Muwatta Malik, Book 20, Hadith 720 (In-Book) ⓘ

Yahya related to me from Malik from Yahya ibn Sa'id that he heard Said ibn al Musayyab say, about the girdle worn by some one in ihram under his clothes, "There is no harm in it if he ties the ends together as a belt." Malik said, "This is what I like most out of what I have heard about the matter."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، أَنَّهُ سَمِعَ سَعِيدَ بْنَ الْمُسَيَّبِ، يَقُولُ فِي الْمِنْطَقَةِ يَلْبَسُهَا الْمُحْرِمُ تَحْتَ ثِيَابِهِ أَنَّهُ لاَ بَأْسَ بِذَلِكَ إِذَا جَعَلَ طَرَفَيْهَا جَمِيعًا سُيُورًا يَعْقِدُ بَعْضَهَا إِلَى بَعْضٍ ‏.‏ قَالَ مَالِكٌ وَهَذَا أَحَبُّ مَا سَمِعْتُ إِلَىَّ فِي ذَلِكَ ‏.‏