Yawning during prayer is disliked; it is a sign of laziness and fullness; and it disrupts concentration. If one is overcome by yawning, they should suppress it as much as possible; if it overwhelms them, they should cover their mouth with their hand or sleeve. Abu Huraira (may Allah be pleased with him) reported that the Prophet (peace be upon him) said: "Yawning in prayer is from the devil, so if one of you yawns, let him suppress it as much as he can." This is found in Sunan al-Tirmidhi 2: 206, and it is classified as good and authentic. See: Bada'i al-Sanai' 1: 215.