He did not ask for proof

Abu Bakr and Muhammad ﷺ had known each other for years before any of this began. They were close in age, both from Quraysh, both merchants, both known in Makkah as men whose word could be trusted.

So when the Prophet ﷺ told him what had happened — that revelation had come to him, that there was one God and the idols of Makkah were nothing — Abu Bakr had a lifetime of knowing him to weigh it against. The reports describe him accepting it immediately, without demanding a sign or asking for time.

The title he became known by, as-Siddiq, means the one who affirms the truth. The reports do not agree on the occasion he was given it — some tie it to this moment, others to his response when the Prophet ﷺ described the Night Journey — so this is a description of a habit rather than the story of a single day. It is not a claim that he never had questions. It is a description of what he did when the person he had trusted his whole life told him something extraordinary.

What followed mattered as much as the moment itself. Abu Bakr was widely connected and widely trusted, and he began speaking to people he knew. Several of the Companions who became well known later are reported to have come to Islam through him. A small community grew partly because one well-connected man did not hesitate.