Musannaf Ibn Abi Shayba 3:6085

Ali bin Hashim narrated to us, from Ibn Abi Layla, from Sadaqah, from Ibn Umar, from the Prophet (peace be upon him), who said: "Indeed, the one who is praying, when he prays, he is conversing with his Lord. So let one of you know whom he is conversing with, and let not some of you raise your voices over others."