Musannaf Ibn Abi Shayba 3:546

Abu Bakr narrated to us, he said: Ibn Idris narrated to us, from ‘Asim ibn Kulayb, from his father, from Wa’il ibn Hujr, that the Prophet (may Allah bless him and grant him peace) sat, so he folded his left [leg] and kept his right [leg] upright, meaning in the prayer.