Muwatta Malik, Book 20, Hadith 12 (USC-MSA)

Yahya related to me from Malik from Nafi that 'Abdullah ibn 'Umar used to disapprove of anybody wearing a belt or girdle while in ihram.

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَكْرَهُ لُبْسَ الْمِنْطَقَةِ لِلْمُحْرِمِ ‏.‏

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also referenced under: Book 20, Hadith 719 (In-Book)