Musnad Ahmad 31:895

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Isma'il ibn Umayyah, from Abu Salamah ibn 'Abd al-Rahman, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) performed i'tikaf in the mosque. He heard them reciting aloud while he was in a small tent of his. He pulled back the curtains and said: "Indeed, all of you are conversing with your Lord, so let not some of you annoy others, and let not some of you raise your voices above others in recitation," or he said: "in prayer."