Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Yawning is from Satan, so when one of you yawns, let him try to suppress it as much as he is able."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3576