Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.145A.

One night, the Prophet, upon whom be peace, passed by Abu Bakr when he was praying in a very low voice, and he passed by ‘Umar who was praying with his voice raised. (Later), when they were together with him, he said, “O Abu Bakr, I passed by you and you were praying in a very low voice.” He said, “O Messenger of Allah, the one who I was praying to could hear me.” And he said to ‘Umar, “O ‘Umar, I passed by you and you were praying with a raised voice.” He said, “O Messenger of Allah, this was to stop the drowsiness and to drive away Satan.” The Prophet, upon whom be peace, said, “O Abu Bakr, raise your voice somewhat. And ‘Umar, lower your voice somewhat.” (Related by Abu Dawud and Ahmad.) If one forgets and recites aloud when he should be silent or vice-versa, there is no blame upon him. If one recalls the correction while he is doing the mistaken act, he may change to the correct way.