ENOfficial
Abu Huraira reported God’s Messenger as saying, “Yawning during prayer comes from the devil, so when one of you yawns he must restrain it as much as possible.” Tirmidhi transmitted it. In another version by him and by Ibn Majah it says, “He should put his hand over his mouth.”
Source: sunnah.com
Other translations
Arabic
وَعَنْ أَبِي هُرَيْرَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «التَّثَاؤُبُ فِي الصَّلَاةِ مِنَ الشَّيْطَانِ فَإِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيَكْظِمْ مَا اسْتَطَاعَ» . رَوَاهُ التِّرْمِذِيّ وَفِي أُخْرَى لَهُ وَلِابْنِ مَاجَهْ: «فَلْيَضَعْ يَدَهُ على فِيهِ»
sunnah.com