كتاب الزهد والرقائق
55
The Book of Zuhd and Softening of Hearts
(9)
Chapter: Saying: "May Allah Have Mercy On You" To One Who Sneezes, And Yawning Is Disliked
(9)
باب تَشْمِيتِ الْعَاطِسِ وَكَرَاهَةِ التَّثَاؤُبِ
Sahih Muslim 2995 a

The son of Abu Said al-Khudri reported on the authority of his father that Allah's Messenger (ﷺ) 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.
حَدَّثَنِي أَبُو غَسَّانَ الْمِسْمَعِيُّ، مَالِكُ بْنُ عَبْدِ الْوَاحِدِ حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، حَدَّثَنَا سُهَيْلُ بْنُ أَبِي صَالِحٍ، قَالَ سَمِعْتُ ابْنًا، لأَبِي سَعِيدٍ الْخُدْرِيِّ يُحَدِّثُ أَبِي عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا تَثَاوَبَ أَحَدُكُمْ فَلْيُمْسِكْ بِيَدِهِ عَلَى فِيهِ فَإِنَّ الشَّيْطَانَ يَدْخُلُ ‏"‏ ‏.‏
Reference : Sahih Muslim 2995 a
In-book reference : Book 55, Hadith 74
USC-MSA web (English) reference : Book 42, Hadith 7130
  (deprecated numbering scheme)