When one of you yawns, he should hold his hand over his mouth, for the devil enters. (Using translation from Abū Dāʾūd 5026)
قَالَ رَسُولُ اللهِ ﷺ إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيَضَعْ يَدَهُ عَلَى فِيهِ فَإِنَّ الشَّيْطَانَ يَدْخُلُ مَعَ التَّثَاؤُبِ