Narrated Abu Salama:

I asked Abu Sa`id, and he was a friend of mine, (about the Night of Qadr) and he said, "We practiced I`tikaf (seclusion in the mosque) in the middle third of the month of Ramadan with the Prophet (ﷺ) . In the morning of the 20th of Ramadan, the Prophet (ﷺ) came and addressed us and said, 'I was informed of (the date of the Night of Qadr) but I was caused to forget it; so search for it in the odd nights of the last ten nights of the month of Ramadan. (In the dream) I saw myself prostrating in mud and water (as a sign). So, whoever was in I`tikaf with me should return to it with me (for another 10-day's period)', and we returned. At that time there was no sign of clouds in the sky but suddenly a cloud came and it rained till rainwater started leaking through the roof of the mosque which was made of date-palm leaf stalks. Then the prayer was established and I saw Allah's Messenger (ﷺ) prostrating in mud and water and I saw the traces of mud on his forehead."