Sahih Bukhari | Volume 9 | Hadith Number 357

Narrated by 'Abdullah bin 'Umar
While the people were at Quba offering the morning prayer, suddenly a person came to them saying, "Tonight Divine Inspiration has been revealed to Allah's Apostle and he has been ordered to face the Ka'ba (in prayers): therefore you people should face it." There faces were towards Sham, so they turned their faces towards the Ka'ba (at Mecca).