Musnad Ahmad 31:198

Yahya narrated to us, Muhammad ibn ‘Amr narrated to us, he said: Abu Salamah ibn ‘Abd al-Rahman narrated to me, he said: We discussed Laylat al-Qadr, and some of the people said: "It rotates throughout the year." So we walked to Abu Sa‘id al-Khudri. I said: "O Abu Sa‘id, did you hear the Messenger of Allah, peace and blessings of Allah be upon him, mention Laylat al-Qadr?" He said: "Yes. The Messenger of Allah, peace and blessings of Allah be upon him, performed i‘tikaf during the middle ten days of Ramadan, and we performed i‘tikaf with him. When we reached the morning of the twentieth, he returned and we returned with him. Laylat al-Qadr was shown to him, then he was made to forget it. He said: 'I was shown Laylat al-Qadr, then I was made to forget it, and I was shown myself prostrating in water and mud. So whoever performed i‘tikaf with me, let him return to his place of i‘tikaf; seek it in the last ten nights, in the odd-numbered ones among them.'" The sky rained upon us at the end of that evening, and half of the mosque was a trellis made of palm branches, so it leaked. By the One who honored him and revealed the Book to him, I saw him leading us in the Maghrib prayer on the night of the twenty-first, and his forehead and the tip of his nose were in the water and the mud.