Sahih Bukhari Volume 1, Book 11, Hadith Number 638

Narated By Abu Sa’id Al-Khudri : A cloud came and it rained till the roof started leaking and in those days the roof used to be of the branches of date-palms. Iqama was pronounced and I saw Allah’s Apostles prostrating in water and mud and even I saw the mark of mud on his forehead.