Sahih Bukhari Volume 6, Book 60, Hadith Number 17.

Narrated By Ibn ‘Umar : While some people were offering morning prayer at Quba’ a man came to them and said, “A Qur’anic Order has been revealed to Allah’s Apostle tonight that he should face the Ka’ba at Mecca (in prayer), so you too should turn your faces towards it.” At that moment their faces were towards Sham (i.e. Jerusalem) (and on hearing that) they turned towards the Ka’ba (at Mecca).