Muwatta Malik, Book 51, Hadith 6 (USC-MSA)

Yahya related to me from Malik from Yahya ibn Said that Abu Qatada al-Ansari said to the Messenger of Allah, may Allah bless him and grant him peace, "I have a lot of hair which comes down to my shoulders, shall I comb it?'' The Messenger of Allah. may Allah bless him and grant him peace, said, "Yes, and honour it." Sometimes Abu Qatada oiled it twice in one day because the Messenger of Allah, may Allah bless him and grant him peace, said to him. "Honour it."

حَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّ أَبَا قَتَادَةَ الأَنْصَارِيَّ، قَالَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم إِنَّ لِي جُمَّةً أَفَأُرَجِّلُهَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ نَعَمْ وَأَكْرِمْهَا ‏"‏ فَكَانَ أَبُو قَتَادَةَ رُبَّمَا دَهَنَهَا فِي الْيَوْمِ مَرَّتَيْنِ لِمَا قَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ وَأَكْرِمْهَا ‏"‏ ‏.‏

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also referenced under: Book 51, Hadith 1738 (In-Book)