Hazrat ‘Aa’ishah (radhiyallahu ‘anha) reports that on the occasion of Mi’raaj, when Rasulullah (sallallahu ‘alaihi wasallam) was taken by night from Musjid Haraam to Musjid Aqsa (and thereafter to the seven heavens), then upon his return, he informed the people of Makkah regarding the miraculous journey that transpired during the night.
The people of Makkah doubted the words of Rasulullah (sallallahu ‘alaihi wasallam) and thus began to ridicule him. Some people even reneged from Islam who had previously believed in Rasulullah (sallallahu ‘alaihi wasallam). Nevertheless, when Hazrat Abu Bakr (radhiyallahu ‘anhu) was informed by certain people (who were trying to provoke him) regarding what had transpired, he responded by saying, “Indeed I fully testify to what Muhammed (sallallahu ‘alaihi wasallam) has said. I have believed him in that which is even more difficult to believe, I believe him when he says that revelation comes to him from the heavens to the earth in a single moment during the night or day (so why should I then doubt him in this matter?).”
It was after this incident that he received the title As-Siddeeq (the most truthful).
(Usdul Ghaabah 3/21 and Seerat-ul-Halabiyyah 1/379)