Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 15
Narrated Ibn 'Umar:
While some people were offering Fajr prayer in the Quba' mosque, some-one came and said, "Allah has revealed to the Prophet Qur'anic instructions that you should face the Ka'ba (while praying) so you too, should face it." Those people then turned towards the Ka'ba.