Musnad Ahmad 27:900

'Attab narrated to us, Abu Hamza—meaning al-Sukkari—narrated to us, from Ibn Abi Layla, from Sadaqah al-Makki, 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 of Ramadan, and a small room made of palm leaves was set up for him therein. He said: He put his head out one day and said: "Indeed, the one who is praying is conversing with his Lord, the Mighty and Majestic, so let one of you look into what he is conversing with his Lord about, and let not some of you raise your voices over others in recitation."