Musannaf Ibn Abi Shayba 3:3766

Waki’ narrated to us, he said: Mis’ar narrated to us, from Hammad, from Mujahid, from Sa’id ibn Jubayr, who said: When he prays cross-legged—Mis’ar said: or as he said—he sits. Then, when he intends to bow or prostrate, he folds his leg.