Musnad Abu Ya'la al-Mawsili 154:144

Abu Khaythama narrated to us, Yazid bin Harun narrated to us, Sa'id bin Ziyad al-Shaybani informed us, Ziyad bin Subayh al-Hanafi narrated to us, he said: I was standing praying towards the House (the Ka'bah), and an old man was beside me. I prolonged the prayer, so I placed my hand on my waist. The old man struck my chest with his hand, a strike he did not spare (effort in). I said to myself, "What did he find objectionable about me?" So I hastened to finish. There was a young man sitting behind him, so I said, "Who is this old man?" He said, "This is Abdullah bin Umar." I sat until he finished, then I said, "O Abu Abd al-Rahman, what did you find objectionable about me?" He said, "Are you the one?" I said, "Yes." He said, "That is the *salb* (placing hands on the waist) in prayer, and the Messenger of Allah, peace and blessings of Allah be upon him, used to forbid it."