The Prophet ﷺ said, "Allah likes sneezing and dislikes yawning, so if someone sneezes and then praises Allah, then it is obligatory on every Muslim who heard him, to say: May Allah be merciful to you (Yar-hamuka-l-lah). But as regards yawning, it is from Satan, so one must try one's best to stop it, if one says 'Ha' when yawning, Satan will laugh at him."  

عَنِ النَّبِيِّ ﷺ إِنَّ اللَّهَ يُحِبُّ الْعُطَاسَ وَيَكْرَهُ التَّثَاؤُبَ فَإِذَا عَطَسَ فَحَمِدَ اللَّهَ فَحَقٌّ عَلَى كُلِّ مُسْلِمٍ سَمِعَهُ أَنْ يُشَمِّتَهُ وَأَمَّا التَّثَاوُبُ فَإِنَّمَا هُوَ مِنَ الشَّيْطَانِ فَلْيَرُدَّهُ مَا اسْتَطَاعَ فَإِذَا قَالَ هَا ضَحِكَ مِنْهُ الشَّيْطَانُ