Sahih Bukhari Volume 8, Book 73, Hadith Number 103.

Narated By Anas bin Malik : Allah’s Apostle visited a household among the Ansars, and he took a meal with them. When he intended to leave, he asked for a place in that house for him, to pray so a mat sprinkled with water was put and he offered prayer over it, and invoked for Allah’s Blessing upon them (his hosts).