When the Prophet (peace be upon him) called to belief in Allah, the slaves of Allah from every tribe answered his call and the first of them was Abu Bakr, the Sideeq  of this nation (may Allah be pleased with him). He supported him in defending Allah’s religion and he called with him to Allah and among those who answered his call were ‘Uthman, Talhah and Sa’d (may Allah be pleased with them).
Another who responded early on to his call was As-Siddeeqah, Khadeejah (may Allah be pleased with her), as did ‘Ali ibn Abi Talib (may Allah be pleased with him) when he was only eight years old. It was said that he was older, while he was under the guardianship of the Messenger of Allah (peace be upon him), he having taken him from his uncle (Abu Talib).
 Sideeq: A title bestowed upon him by the Prophet (peace be upon him) due to the fact that he believed the Prophet’s story of the Night Journey (Al-Isra’) and the Ascension to the heavens (Al-Mi’raj) when others did not.
Source: At-Tamimi, Muhammad Ibn ‘Abdul Wahhab, Abrdiged Biography of Prophet Muhammad Peace be upon him (Mukhtasar Seerah Ar-Rasool), Darussalam, Riaydh, Saudi Arabia, 2003. p.113.