Musannaf Ibn Abi Shayba 3:165

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Mughira, who said: I said to Ibrahim, "Did Abdullah [ibn Mas'ud] use to clasp his hands together and place them between his thighs, and spread his forearms upon his thighs when he bowed?" He said, "Yes." I said, "I do not do that." He said, "Indeed, Umar used to clasp his palms upon his knees."