It is reported that Al-Sha'bī - Allāh have mercy on him - said:

Allāh the Exalted distinguished Abū Bakr Al-Ṣiddīq - Allāh be pleased with him - with four qualities He did not distinguish anyone else with: He called him Al-Ṣiddīq (the true believer), and He never named anyone else Al-Ṣiddīq; [Abū Bakr] was the companion of Allāh's Messenger - praise and peace of Allāh be upon him - in the cave, and his companion during the Migration (Al-Hijrah); and Allāh's Messenger - praise and peace of Allāh be upon him - told him to lead the prayer in the presence of the rest of the Muslims.

Abu Bakr Al-Daynūrī, Al-Mujālasah wa Jawāhir Al-'Ilm article 2815.