Musnad Ahmad 265:4

Anas ibn Iyad Abu Damrah narrated to us, he said: Al-Dahhak ibn Uthman narrated to me, from Abu al-Nadr, the freed slave of Umar ibn Ubayd Allah, from Busr ibn Sa’id, from Abdullah ibn Unays, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "I was shown the Night of Decree (Laylat al-Qadr), then I was made to forget it. And it was shown to me that on its morning, I would be prostrating in water and mud." So it rained on the night of the twenty-third, and the Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer. When he finished, the traces of water and mud were on his forehead and his nose.