Sayyidah Umm Hani (may Allah be pleased with her) said that when the Messenger of Allah (peace and blessings be upon him) was taken on the Night Journey (Mi'raj), he said: "I want to go out to Quraysh and tell them." So he (peace and blessings be upon him) told them, and they denied him, while Abu Bakr (may Allah be pleased with him) affirmed him as truthful; so from that day he came to be called "al-Siddiq".
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 15