Sahih Bukhari
Call to Prayers (Adhaan)
Volume: 001 / Book Number: 011 / Hadith Number: 638
Narrated 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.