It is to stretch during prayer, which is disliked; because it is a sign of laziness. Abu Huraira (may Allah be pleased with him) reported: "The Messenger of Allah (peace be upon him) prohibited a man from stretching during prayer, or in front of women, except with his wife or his female slaves." This was reported by Al-Daraqutni in Al-Af'al and weakened by Al-Suyuti in Al-Jami' Al-Saghir 6: 350. Al-Tahanawi said in I'la Al-Sunan 5: 149: and the analogy supports it, and this is what the scholars have said, and it is a sign of acceptance. See: Al-Bada'i 1: 215, and Al-Tabeen 1: 163.