Sayyiduna Abu Sa’id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: When one of you yawns during prayer, he should suppress it as much as possible; if he cannot suppress it, then he should place his hand over his mouth.
Hadith Referenceالمنتقى ابن الجارود / كتاب الصلاة / 221