Narrated Abu Hurairah:
that the Messenger of Allah (ﷺ) said: “The sneeze is from Allah and the yawn is from Ash-Shaitan. So when one of you yawns let him cover his mouth with his hand. For when he says ‘Ah, Ah’ Ash-Shaitan laughs from inside his opening.” [And indeed Allah loves the sneeze and He dislikes the Yawn, so when a man says Ah, Ah when yawning, indeed Ash-Shaitan laughs from inside his opening.”]
||: Sahih (Darussalam)
|| : Vol. 5, Book 41, Hadith 2746
|| : Book 43, Hadith 2970
Cover your mouth while you yawn, it is a sunnah and moreover none of us want the shaitaan to laugh because of us.