Musnad Ahmad

Nafi’ (may Allah have mercy on him) says that once Abdullah ibn Umar (may Allah be pleased with them both) set out with the intention of Hajj. When he entered into the state of Ihram, he began to feel severe cold on his head. I covered him with a cap. When he became aware, he said: "What have you put on me?" I said, "It is a cap." He said: "I have already told you that the Noble Prophet (peace and blessings of Allah be upon him) forbade us from wearing it in the state of Ihram, yet you still put it on me?"