Sahih Bukhari Volume 6, Book 60, Hadith Number 18.

Narrated By Ibn Umar : While some people were offering Fajr prayer at Quba’ (mosque), some-one came to them and said, “Tonight some Qur’anic Verses have been revealed to the Prophet and he has been ordered to face the Ka’ba (at Mecca) (during prayers), so you too should turn your faces towards it.” At that time their faces were towards Sham (Jerusalem) so they turned towards the Ka’ba (at Mecca).