Musannaf Ibn Abi Shayba 3:1304

Hafs narrated to us, from Imran bin Za'idah bin Nashit, from his father, from Abu Khalid al-Walibi, from Abu Hurairah, who said: When the Prophet, peace and blessings of Allah be upon him, would stand at night, he would lower [his voice] at times, and raise [it] at times.