Musnad Ahmad 31:272

Waki’ narrated to us, Sufyan narrated to us, from Suhayl ibn Abi Salih, from Ibn Abi Sa’id al-Khudri, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you yawns during prayer, let him suppress it as much as he can, for the devil enters through his mouth."