Abu Hurayra reported that the Prophet, may Allah bless him and grant him peace, said, "When one of you yawns, he should repress it as much as possible."  

عَنِ النَّبِيِّ ﷺ قَالَ إِذَا تَثَاءَبَ أَحَدُكُمْ فَلْيَكْظِمْ مَا اسْتَطَاعَ