Musnad Ahmad 31:49

Sufyan narrated to us, Muhammad ibn ‘Amr narrated to us, from Abu Salamah, and Ibn Abi Labid, from Abu Salamah, [who said]: I heard Abu Sa‘id, and Ibn Jurayj from Sulayman al-Ahwal, from Abu Salamah, from Abu Sa‘id: He [the Prophet] performed i‘tikaf during the middle ten days, and we performed i‘tikaf with him—meaning the Prophet, peace and blessings of Allah be upon him. When it was the morning of the twentieth, he passed by us while we were moving our belongings. He said: "Whoever was performing i‘tikaf, let him return to his place of i‘tikaf, for I saw this night and then I forgot it, and I saw myself prostrating in water and mud, and the roof of the mosque was made of palm branches, and the sky poured rain." I saw the Messenger of Allah, peace and blessings of Allah be upon him, and upon his nose and his forehead was the trace of water and mud.