Musannaf Ibn Abi Shayba 3:262

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from ‘Ata’ ibn al-Sa’ib, from Salim al-Barrad, who said: We came to Abu Mas’ud in his house and said to him: "Teach us the prayer of the Prophet (ﷺ)." So he prayed. When he prostrated, he kept his thighs apart.