It is narrated from Muslim bin Jundab, he states that a man came and was asking Ibn Umar (may Allah be pleased with them both) while I was with them. That man said: In the state of Ihram, can I take both ends of my sheet to the opposite sides behind me and tie a knot on them? Abdullah bin Umar (may Allah be pleased with them both) replied: You must not tie a knot at all.
Hadith Referenceمسند الإمام الشافعي / كتاب الحج / 843
Hadith Takhrijاسناده حسن اخرجه البيهقي ۔ وفى المعرفة السنن والآثار له (2835) وابن ابي شيبة (15438)۔