Titles: Amir al-Shakirin
“Amir al-Shakirin” was another title of Hazrat Abu Bakr (ra), meaning the leader of the grateful. Hazrat Abu Bakr Siddiq (ra) was known as Amir al-Shakirin because of how grateful he was. It is recorded in Umdat al-Qari that Hazrat Abu Bakr (ra) would be called by the title of Amir al-Shakirin.
(Umdatul-Qari, Kitab Bad‘ al-Khalq, Bab Manaqib al-Muhajirin wa Fadlihim, Vol. 16, p. 260, Dar Ihya al-Turath al-Arabi, Beirut, 2003)