Musnad Ahmad 27:401

Yazid narrated to us, Sa'id ibn Ziyad al-Shaybani informed us, Ziyad ibn Subayh al-Hanafi narrated to us, he said: I was standing praying towards the House (the Kaaba), and an old man was beside me. I prolonged the prayer, and I placed my hand on my waist. The old man struck my chest with his hand with a strike that did not spare (effort). I said to myself: What did he find objectionable in me? So I hastened to finish. Then there was a young man sitting behind him. I said: Who is this old man? He said: This is Abdullah ibn Umar. I sat until he finished, then I said: O Abu Abd al-Rahman, what did you find objectionable in 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, may Allah bless him and grant him peace, used to forbid it."