Musannaf Ibn Abi Shayba 21:546

Ali ibn Hakim narrated to us, from Ibn Abi Layla, from Sadaqah, from Ibn Umar, from the Prophet (may Allah bless him and grant him peace), who said: "Indeed, the one who is praying is conversing with his Lord, so let him know what he is conversing with Him about, and let not some of you raise your voices over others."