Musannaf Ibn Abi Shayba 3:162

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Qatan narrated to us, from Abu Ishaq, from ‘Asim ibn Damrah, from ‘Ali, who said: When you bow, if you wish, you may say [do] like this, and if you wish, you may place your hands upon your knees, and if you wish, you may say [do] like this—meaning you clasp [your hands together].