Musannaf Ibn Abi Shayba 3:2213

Narrated to us Sa'id ibn Ziyad and Waki' from Ziyad ibn Subayh al-Hanafi, who said: I prayed beside Ibn Umar, and I placed my hand upon my waist. When he finished the prayer, he said: "This is the 'salb' (placing hands on the waist) in prayer; the Messenger of Allah (peace be upon him) used to forbid it."