Musnad Ahmad 27:479

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Ma'mar, from Sadaqah al-Makki, from Abdullah ibn Umar, that the Prophet (peace and blessings of Allah be upon him) performed I'tikaf and addressed the people, saying: "Verily, when one of you stands in prayer, he is conversing with his Lord. So let one of you know what he is conversing with his Lord about, and let not some of you raise your voices over others in recitation during prayer."