Muwatta Malik, Book 46, Hadith 3 (USC-MSA)

Yahya related to me from Malik that he heard that the Messenger of Allah, may Allah bless him and grant him peace, said, "I have left two matters with you. As long as you hold to them, you will not go the wrong way. They are the Book of Allah and the Sunna of His Prophet."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ تَرَكْتُ فِيكُمْ أَمْرَيْنِ لَنْ تَضِلُّوا مَا تَمَسَّكْتُمْ بِهِمَا كِتَابَ اللَّهِ وَسُنَّةَ نَبِيِّهِ ‏" ‏ ‏.‏

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also referenced under: Book 46, Hadith 1628 (In-Book)