Sahih Bukhari Volume 6, Book 60, Hadith Number 13.

Narrated By Al-Bara : The Prophet prayed facing Bait-ulMaqdis (i.e. Jerusalem) for sixteen or seventeen months but he wished that his Qibla would be the Ka’ba (at Mecca). (So Allah Revealed (2.144) and he offered ‘Asr prayers(in his Mosque facing Ka’ba at Mecca) and some people prayed with him. A man from among those who had prayed with him, went out and passed by some people offering prayer in another mosque, and they were in the state of bowing. He said, “I, (swearing by Allah,) testify that I have prayed with the Prophet facing Mecca.” Hearing that, they turned their faces to the Ka’ba while they were still bowing. Some men had died before the Qibla was changed towards the Ka’ba. They had been killed and we did not know what to say about them (i.e. whether their prayers towards Jerusalem were accepted or not). So Allah revealed: “And Allah would never make your faith (i.e. prayer) to be lost (i.e. your prayers offered (towards Jerusalem). Truly Allah is Full of Pity, Most Merciful towards mankind.” (2.143)