Muwatta Malik, Book 51, Hadith 1 (USC-MSA)

Yahya related to me from Malik from Abu Bakr ibn Nafi from his father Nafi from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, ordered the moustache to be trimmed and the beard to be left.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي بَكْرِ بْنِ نَافِعٍ، عَنْ أَبِيهِ، نَافِعٍ عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم أَمَرَ بِإِحْفَاءِ الشَّوَارِبِ وَإِعْفَاءِ اللِّحَى ‏.‏

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also referenced under: Book 51, Hadith 1733 (In-Book)