Hadith: When any of you yawns, let him put his hand over his mouth, for indeed, the devil enters

عَن أَبِي سَعِيدٍ الْخُدْرِيِّ رضي الله عنه قَالَ: قَالَ رَسُولُ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ:
«إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيُمْسِكْ بِيَدِهِ عَلَى فِيهِ، فَإِنَّ الشَّيْطَانَ يَدْخُلُ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 2995]

Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) said:
"When any of you yawns, let him put his hand over his mouth, for indeed, the devil enters."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 2995]

Explanation

The Prophet (may Allah's peace and blessings be upon him) instructed anyone who yawns, opening his mouth due to laziness, fullness, or similar reasons, to place his hand over his mouth to close it. This is because the devil enters if the mouth is left open, and placing the hand prevents him from entering.