Mishkat al-Masabih #928

Abu Sa'id reported God’s Messenger as saying, “When one of you yawns during prayer he must restrain it as much as possible, for the devil gets in.” Muslim transmitted it. In a version by Bukhari from Abu Huraira he said, “When one of you yawns during prayer he must restrain it as much as possible and not say ‘Ha’, for that comes from the devil who is laughing at him.”(Or “at it”, i.e. at this action).