Musannaf Ibn Abi Shayba 21:643

Bakr ibn Abd al-Rahman narrated to us, he said: Isa ibn al-Mukhtar narrated to us, from Muhammad, from al-Hakam, from Mujahid, regarding this verse: "And do not recite [your] prayer aloud or whisper it," he said: That is regarding supplication (du'a) and asking (mas'alah).