What is the reason for the Messenger, may Allah bless him and grant him peace, being named "the Truthful, the Trustworthy" (Al-Sadiq Al-Amin), and what is the story that proves this?

The Prophet, may Allah's prayers and peace be upon him, was known for his truthfulness and trustworthiness, so much so that he was called "the Trustworthy" (Al-Amin) even in the pre-Islamic era (Jahiliyyah). His staunchest enemies, such as Abu Jahl, bore witness to this, acknowledging his truthfulness and trustworthiness. However, his arrogance and stubbornness prevented him from believing in him. This confirms his appellation as "the Truthful, the Trustworthy" (Al-Sadiq Al-Amin).