It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "When one of you yawns, he should place his hand over his mouth, for the devil enters the mouth."
Hadith Referenceالادب المفرد / كتاب العطاس والتثاؤب / 949
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : صحيح مسلم ، الزهد و الرقائق ، ح : 2995»