Sahih Bukhari Volume 006, Book 060, Hadith Number 020.
Narrated By Ibn Umar : While some people were at Quba (offering) morning prayer, a man came to them and said, "Last night Qurâ€™anic Verses have been revealed whereby the Prophet has been ordered to face the Ka'ba (at Mecca), so you too should face it." So they, keeping their postures, turned towards the Ka'ba. Formerly the people were facing Sham (Jerusalem) (Allah said):
"And from whence-so-ever you start forth (for prayers), turn your face in the direction of the Sacred Mosque of Mecca (Al-Masjid-ul-Haram), and whence-so-ever you are, turn your face towards it (when you pray)" (2.150)