Musnad Ahmad 27:1385

Waki’ narrated to us, Sa’id ibn Ziyad narrated to us, from Ziyad ibn Subayh al-Hanafi, who said: I prayed beside Ibn Umar, and I placed my hand upon my waist. He struck my hand, and when he finished the prayer, he said: "This 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."