It is narrated from Sayyiduna Abu Sa'id Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When any one of you yawns, he should cover his mouth with his hand, because (if the mouth is open) Satan enters."
And the evidence that the Prophet, peace and blessings be upon him, forbade covering the mouth in prayer except in the case of yawning, is that the Prophet, peace and blessings be upon him, commanded covering the mouth when yawning.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب الأفعال المكروهة فى الصلاة التى قد نهي عنها المصلي / 919