Hadith 4737
وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيُّ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ : «إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيَمْسِكْ بِيَدِهِ عَلَى فَمه فإِنَّ الشيطانَ يدخلُ» . رَوَاهُ مُسلم
It is narrated from Abu Sa'id Al-Khudri, may Allah be pleased with him, that the Messenger of Allah, peace and blessings be upon him, said: "When one of you yawns, he should place his hand over his mouth, for the devil enters (into the mouth)." Narrated by Muslim.