Muwatta Malik 616

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ جَعْفَرِ بْنِ مُحَمَّدِ بْنِ عَلِيٍّ، عَنْ أَبِيهِ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، ذَكَرَ الْمَجُوسَ فَقَالَ مَا أَدْرِي كَيْفَ أَصْنَعُ فِي أَمْرِهِمْ فَقَالَ عَبْدُ الرَّحْمَنِ بْنُ عَوْفٍ أَشْهَدُ لَسَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏ "‏ سُنُّوا بِهِمْ سُنَّةَ أَهْلِ الْكِتَابِ ‏"‏ ‏.‏

Yahya related to me from Malik from Jafar ibn Muhammad ibn Ali from his father that Umar ibn al-Khattab mentioned the magians and said, "I do not know what to do about them." Abd ar-Rahman ibn Awf said, "I bear witness that I heard the Messenger of Allah, may Allah bless him and grant him peace, say, 'Follow the same sunna with them that you follow with the people of the Book

Reference

Muwatta Malik 616

In-book

Book 17, Hadith 39