Musnad Ahmad 27:1667

Ubaydah narrated to us, Muhammad ibn Abd al-Rahman ibn Abi Layla narrated to us, from a man called Sadu'—and in one version, Sadaqah—from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) performed I'tikaf during the last ten days. He said: So a house of palm fronds was built for him. He said: Then he put his head out of it one night and said: "O people, indeed the one who prays, when he prays, he is only conversing with his Lord, Blessed and Exalted is He. So let him know what he is conversing with Him about, and let not some of you raise your voices over others."