Opposing his [the Prophet's] command and changing his sunna is misguidance and innovation. It is threatened by Allah with utter bereftness and punishment.

Allah says, {"Let those who oppose his command beware of a trial that might strike them or a painful punishment that might befall them,"} (Sura an-nûr 24, verse 63) and {"Whoever splits from the Messenger after the guidance is clear to him and follows other than the path of the believers, We shall entrust him to what he has turned to."} (Sura an-nisâ 4, verse 114)

Abu Hurayra said that once the Messenger of Allah went to visit some graves and, while there, described his community saying, "Some men will be driven away from my Basin as a stray camel is driven off. I will call to them, 'Come here! Come here!' It will be said, 'They made changes after you.' Then I will say, 'Get away! Get away!'" (Muslim, Malik, Abu Dawûd and an-Nasâ'i )

Anas related that the Prophet said,
"Anyone who dislikes my sunna is not of me." (Muslim and Al-Bukhâri)

The Prophet said, "Anyone who adds something to my commands which is not part of them is a renegade." (Muslim and Al-Bukhâri)

Ibn Abi Rafi` related from his father that the Prophet said, "Do not let any of you be found reclining on his bed when he hears an injunction from me which is from among those things which I myself have commanded or forbidden so that he says, 'I do not know. We only follow what we find in the Book of Allah.'" He added a version of the hadith from al-Miqdam, "What the Messenger of Allah makes unlawful is like what Allah makes unlawful."
(Abu Dawûd, at-Tirmidhi and Ibn Majah)

Once when some writing on a shoulder-blade was brought to him, the Prophet said, "People are stupid enough to dislike, what their Prophet has brought them, preferring someone else to their Prophet or another book to their book." So it was revealed, { or is it not enough for them that We sent down the Book on them which is recited to them? } (Sura al-`Ankabut (29) verse 51)
(Abu Dawûd, ad-Darimi, Ibn Jarir and others )

The Prophet , may Allah bless him and grant him peace said:
"Those who go to extremes in speech will be destroyed."
(Muslim via Ibn Mas`ûd)

Abu Bakr as-Siddiq said:
"I have not omitted doing anything that the Messenger of Allah used to do.
I feared that I would deviate if left out any of his commands."

Imam Al-Ghazali wrote*: I proceed to enlighten you, who are the most self-righteous of those who reject belief, and you, who are the most immoderate of the thoughtless unbelievers.

I am no longer obliged to remain silent, because the responsibility to speak, as well as warn you, has been imposed upon me by your persistent straying from the clear truth, and by your insistence upon fostering evil, flattering ignorance, and stirring up opposition against him who, in order to conform to the dictates of knowledge, deviates from custom and the established practice of men. In doing this he fulfils Allah's prescriptions for purifying the self and reforming the heart, thus somewhat redeeming a life, which has already been dissipated in despair of prevention and remedy, and avoids by it the company of him whom the Lawgiver - Prophet Muhammad - described when he said:
"The most severely punished of all men on the day of resurrection will be a learned man whom Allah has not blessed with His knowledge."

For, by my life, there is no reason for your abiding arrogance except the malady which has become an epidemic among the multitudes. That malady consists in not discerning this matter's importance, the gravity of the problem, and the seriousness of the crisis; in not seeing that life is waning and that what is to come is close at hand, that death is imminent but that the journey is still long, that the provisions are scanty, the dangers great, and the road blocked.
The perceptive know that only knowledge and works devoted to Allah avail.

To tread the crowded and dangerous path of the Hereafter with neither guide nor companion is difficult, tiring, and strenuous. The guides for the road are the learned men who are the heirs of the Prophet , but the times are void of them now and only the superficial are left, most of whom have been lured by iniquity and overcome by Satan.

Everyone of them was so wrapped up in his immediate fortune that he came to see good as evil and evil as good, so that the science of religion disappeared and the torch of the true faith was extinguished all over the world.

They duped the people into believing that there was no knowledge except such ordinances of government as the judges use to settle disputes when the mob dots; or the type of argument which the vainglorious displays in order to confuse and refute; or the elaborate and flowery language with which the preacher seeks to lure the common folk.

They did this, because apart from these three, they could find no other ways to snare illegal profit and gain the riches of the world. On the other hand the science of the path of the hereafter, which our forefathers trod and which includes what Allah in His Book called law, wisdom, knowledge, enlightenment, light, guidance, and righteousness, has vanished from among men and been completely forgotten.

Since this is a calamity afflicting religion and a grave crisis overshadowing it, I have therefore deemed it important to engage in the writing of this book;
to revive the science of religion, to bring to light the exemplary lives of the departed imams, and to show what branches of knowledge the prophets and the virtuous fathers regarded as useful.

I have divided the work into four parts or quarters. These are:
- the Acts of Worship,
- the Usages of Life,
- the Destructive Matters in Life, and
- the Saving Matters in Life.

fn: From his Introduction to the Ihya' `Ulum al-Din

Imam Al-Ghazali wrote on the original meaning of fiqh *:
The first term to be affected was fiqh [Arabic root: fqh, to be wise, to understand], whose meaning has been tampered with not so much by alteration and change, but by limitation. ...
But in the early period of Islam the term jurisprudence (fiqh) was applied to the science of the path of the hereafter and the knowledge of the subtle defects of the soul (or: the science of the hereafter and the nature of the heart), the influences of which render works corrupt, the thorough realization of the inferiority of this life, the urgent expectation of bliss in the hereafter, and the domination of fear in the heart.
This is indicated by the words of Allah when He says:
{�... that they may instruct themselves in their religion,
and may warn their people when they come back to them.} (Sura 9-122) Jurisprudence (fiqh) is therefore that which brings about such a warning and such a fear rather than details of (for example) ordinary divorce or divorce through li'an, or salam contracts, and hire, rental and lease (etc) which produce neither warnings nor fear.
On the contrary to devote oneself exclusively to these things hardens the heart and removes from it all fear which is exactly what we now see in those who have so devoted themselves. Allah says thus: { Hearts have they with which they understand not. } (Sura 7-179) having had in mind the meaning of faith (iman), not of legal opinions. Upon my life the word fiqh (discernment), now used for jurisprudence, and the word fahm (understanding) are nothing but two names for the same thing. At the present time, however, they are used both in their earlier and also their newer significance.
...
The Prophet    said: “Shall I tell you who is the profoundly discerning man? ...
It is he who has not induced people to dispair of the mercy of Allah; nor made them feel safe (rather than urge them to repent) during the period of respite, which Allah – out of His patience – gives unto man; nor made them lose hope in the spirit of God; nor discarded the Qur'an in favour of something else.”

When Anas ibn Malik related the following words of the Prophet  :
“I prefer sitting in the company of men who praise Allah from the sunset until sunrise to the setting free of four slaves,” he turned to Yazid al-Raqahi and Ziyad al-Numayri and said: “Our meetings of invocation dhikr were different from your present gatherings in which one of you delivers his sermons before his friends and recites traditions. We used to sit and ponder over the articles of faith, study the meaning of the Qur'an, enlighten ourselves in matters of religion and enumerate the blessings of Allah upon us.”
→ Hence the process of studying the meaning of the Qur'an and of enumerating the blessings of Allah was called enlightenment.

The second term to have been altered is the science of religion (al-`ilm) which used to be applied to the knowledge of God, His miracles, and His works among His servants and creatures. When, therefore (the Caliph) `Umar    died, Ibn Mas`ud  exclaimed: “Verily nine-tenths is the science of religion (al-`ilm) has passed away. He thus designated this knowledge as the science, using the definite article, and then explained it as the knowledge of Allah.”

from: *Al-Ghazali; The Book Of Knowledge,
Idara Isha'at-E-Diniyat, New Delhi (India); p.77