He traced it back to the Prophet ﷺ, "When one of you yawns, let him place his hand over his mouth."
حدثنا سفيان عن العلاء عن أبيه
يرفعه إذا تثاءب أحدكم يضع يده على فيه
Its isnad is sahih. Muslim also related it, 2:391, in similar wording at greater length, via Ismail bin Jafar, from Ala, with this same isnad. Bukhari also related it, 10:505, with a similar meaning but at greater length than this, via Ibn Abu Dhib, from Saeed Maqburi, from his father, from Abu Hurairah. It will come again through these three routes at 9151, 9526, and 10706.