It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Prophet Muhammad (peace and blessings be upon him) said: Yawning in prayer is from Satan, so if any one of you feels the urge to yawn, he should try to restrain it as much as possible.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المكروهة فى الصلاة التى قد نهي عنها المصلي / 920