Sahih Bukhari Volume 001, Book 004, Hadith Number 147.
Narated By 'Abdullah bin 'Umar : People say, "Whenever you sit for answering the call of nature, you should not face the Qibla or Bait-ulMaqdis (Jerusalem)." I told them. "Once I went up the roof of our house and I saw Allah's Apostle answering the call of nature while sitting on two bricks facing Bait-ul-Maqdis (Jerusalem) (but there was a screen covering him. ' (FatehAl-Bari, Page 258, Vol. 1).