Muwatta Malik, Book 51, Hadith 1735 (In-Book) ⓘ

Yahya related to me from Malik that Ziyad ibn Sad heard Ibn Shihab say, "The Messenger of Allah, may Allah bless him and grant him peace, let his hair hang down his forehead as Allah willed, and afterwards he parted it." Malik said, "There is no harm in a man's looking at the hair of his son's wife or the hair of his wife's mother."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زِيَادِ بْنِ سَعْدٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ سَمِعَهُ يَقُولُ سَدَلَ رَسُولُ اللَّهِ صلى الله عليه وسلم نَاصِيَتَهُ مَا شَاءَ اللَّهُ ثُمَّ فَرَقَ بَعْدَ ذَلِكَ ‏.‏ قَالَ مَالِكٌ لَيْسَ عَلَى الرَّجُلِ يَنْظُرُ إِلَى شَعَرِ امْرَأَةِ ابْنِهِ أَوْ شَعَرِ أُمِّ امْرَأَتِهِ بَأْسٌ ‏.‏