Muwatta Malik, Book 20, Hadith 722 (In-Book)

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say that a man in ihram should not veil anything above his chin.

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ مَا فَوْقَ الذَّقَنِ مِنَ الرَّأْسِ فَلاَ يُخَمِّرْهُ الْمُحْرِمُ ‏.‏

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also referenced under: Book 20, Hadith 13 (USC-MSA)