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 c

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 while engaged in prayer, he should try to restrain so far as it lies in his power, since it is the Satan that enter therein.

حَدَّثَنِي أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ سُفْيَانَ، عَنْ سُهَيْلِ بْنِ أَبِي صَالِحٍ، عَنِ ابْنِ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِذَا تَثَاوَبَ أَحَدُكُمْ فِي الصَّلاَةِ فَلْيَكْظِمْ مَا اسْتَطَاعَ فَإِنَّ الشَّيْطَانَ يَدْخُلُ ‏"‏ ‏.‏
Reference : Sahih Muslim 2995 c
In-book reference : Book 55, Hadith 76
USC-MSA web (English) reference : Book 42, Hadith 7132
  (deprecated numbering scheme)