Musnad Ahmad 30:2403

Yahya ibn Sa'id narrated to us, from Ibn Abi Dhi'b, Sa'id ibn Abi Sa'id narrated to me, and Hajjaj said: Ibn Abi Dhi'b informed us, from Sa'id ibn Abi Sa'id al-Maqburi, from his father, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah the Almighty and Majestic loves sneezing and dislikes yawning. So, whoever sneezes and praises Allah, it is a duty upon whoever hears him to say: 'May Allah have mercy on you' (Yarhamuk Allah). And if one of you yawns, let him suppress it as much as he can, and let him not say: 'Ah, ah,' for when one of you opens his mouth, the devil laughs at him—or because of him." Hajjaj said in his narration: "As for yawning, it is only from the devil."