Musannaf Ibn Abi Shayba 3:2495

Abu Bakr narrated to us, he said: Abbad ibn al-Awwam narrated to us, from Hisham, from al-Hasan, that he used to dislike for a man to lean against a wall during the obligatory prayer, unless there was an ailment. And he did not see any harm in it during the voluntary prayer.