The Advice of ʿUmar ibn ʿAbd al-ʿAzīz Concerning the Incumbency of the Sunnah and Following the Salaf
An enlightening reminder about the obligation of following the Sunnah of the Prophet (ṣallallāhu ʿalayhi wa-sallam), and the obligation to interpret the Sunnah with the interpretation of the Salaf al-Ṣāliḥ.
Then know that there cannot ever be any innovation, except that a proof against it has already passed, and there is a warning in it.
From Shihaab Ibn Khiraash  who said: ʿUmar Ibn ʿAbd al-ʿAzīz (d.101H) – raḥimahullāh- wrote to a man,
“May peace be upon you, to proceed:
So verily I advise you to fear Allāh, and to make His Command the objective, and to follow the Sunnah of His Messenger, and to leave off what people invented after him (ṣallallāhu ʿalayhi wa-sallam), from whatever is confirmed as his Sunnah, and his supply will suffice you.
Then know that there cannot ever be any innovation, except that a proof against it has already passed, and there is a warning in it. So adhere strictly to the Sunnah, for verily the Sunnah is – with the permission of Allāh – a protection for you. Verily the one who practices the Sunnahis one who indeed knows what opposes it from mistakes and error, and foolishness, and deep thorough study.
So be pleased with for yourself what the people (i.e. the Salaf al-Ṣāliḥ) were pleased with for themselves, for verily they stopped upon knowledge, and they sufficed with penetrating insight, and they were the strongest in studying the matters, and the virtue that was in them, in that case, they were the most adequate; since they (the Salaf al-Ṣāliḥ – Righteous Predecessors) have already preceded. So indeed the guidance is not what you are upon, rather they preceded you to it. Therefore you say, he innovated a newly invented matter after them, so no one invents something, except that he opposes their path, and considers himself to be better than them. Indeed whatever they spoke was sufficient, and whatever they described was satisfying. So whatever is below them is deficient, and there is no good above them. Indeed people fell short of their mark, so they became dull, and others were too ambitious, so they went into extremes, and verily they (the Salaf) were in between the two, so they were upon the straight guidance.” 
From Mālik  (d.179H) – raḥimahullāh - who said: ʿUmar Ibn ʿAbd al-ʿAzīz used to say, “The Messenger of Allāh (ṣallallāhu ʿalayhi wa-sallam) and the caretakers of the affairs (wulātul-umoor) enacted Sunnahs, taking to them is affirmation (tasdeeq) of the Book of Allāh, and the completion of obedience to Allāh, and strength upon the Religion of Allāh. Whoever seeks guidance through them will be guided, and whoever seeks aid through them will be victorious, and whoever opposes them; he follows a path other than the Path of the Believers, and the close allies (wulāt) of Allāh are not with him, and his destination is Hell, and an evil end.” 
Mālik said, “I was amazed at ʿUmar when he obligated the Fire for such a person.”
And he added to his statement: “…upon the Religion of Allāh…”, “And it is not for anyone to change it, nor to replace it, nor to oppose anything from it.” 
 He is Shihaab Ibn Khiraash Ibn Hawshab ash-Shaybaanee, from Kūfah. He moved to Shaam, and lived in ar-Ramlah inside of Palestine, and he died there. He was sadooq (truthful), a person of the Sunnah. Refer to Tahdhīb al-Kamāl(12/568-572).
 Related by Ibn Baṭṭah in al-Ibānah (1/321-322), and by al-Lālikaa‘ee in Sharḥ Uṣūl al-Iʿtiqād(no. 16)
 This is Imām Mālik Ibn Anas, Imām Daarul-Hijrah(the scholar of the place of migration, i.e. al-Madīnah)
 The proof for this is the statement of Allāh, “And whosoever contraḍīcts and opposes the Messenger, after the right path has been shown clearly to him, and follows other than the Path of the Believers; We shall keep him upon the path he has chosen, and roast him in Hell. What an evil destination!” [Sūrah Sūrah al-Nisāʾ 4:115]
 Related by Ibn Baṭṭah in al-Ibānah (1/352-353)