Which companion of the Prophet was nicknamed 'As-Siddiq'?

Abu Bakr was the companion of Prophet Muhammad who earned the title As-Siddiq, meaning "The Truthful" or "The Testifier to the Truth." He received this title because he immediately and unhesitatingly believed the Prophet's account of the Night Journey and Ascension (Isra wal-Mi'raj), when Muhammad reported being transported from Makkah to Jerusalem and then ascending through the heavens in a single night. While many in Makkah doubted the account, Abu Bakr affirmed it without hesitation.

Abu Bakr was the first adult male to accept Islam and remained the Prophet's closest companion throughout his life. He later became the first Caliph (Khalifah) of Islam after the Prophet's death.

The other companions listed hold different titles: Umar ibn al-Khattab is known as Al-Faruq (The Distinguisher between truth and falsehood). Uthman ibn Affan is known as Dhun-Nurayn (The Possessor of Two Lights, having married two of the Prophet's daughters). Ali ibn Abi Talib is known as Asadullah (The Lion of Allah).