Abu Hurayrah (RAA) said that the Prophet (SAW) said: “Allah likes the act of sneezing and dislikes the act of yawning, so if any one of you sneezes and praises Allaah (says ‘al-hamdu Lillaah’), it is a duty on every Muslim who hears him to say to him, ‘Yarhamuk Allaah (may Allaah have mercy on you)’. As for yawning, it is from the Shaytaan, so if any of you feels the urge to yawn, he should suppress it as much as he can, for when any one of you yawns, the Shaytaan laughs at him”. Sahih al-Bukhaari, 10/505

The son of Abu Said al−Khudri reported on the authority of his father that Allah’s Messenger (SAW) said, “When one of you yawns, he should keep his mouth shut with the help of his hand, for it is the devil that enters therein”. Sahih Muslim (#7130)