ENOfficial
Abu Sa'id reported that the Prophet, may Allah bless him and grant him peace, said, "When one of you yawns, he should put his hand over his mouth. Otherwise Shaytan might enter it."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، قَالَ: حَدَّثَنَا خَالِدٌ، قَالَ: حَدَّثَنَا سُهَيْلٌ، عَنِ ابْنِ أَبِي سَعِيدٍ، عَنْ أَبِي سَعِيدٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ: إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيَضَعْ يَدَهُ بِفِيهِ، فَإِنَّ الشَّيْطَانَ يَدْخُلُ فِيهِ.
sunnah.com