Musannaf Ibn Abi Shayba 3:5709

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hisham ibn ‘Urwah, from his father, from ‘Aishah, regarding His saying: "And do not recite [your] prayer [too] loudly or [too] quietly," she said: "It is regarding supplication (du’a)."