Abu Bakr
Imam Jafar al Sadiq famously narrated how the title Siddiq was given to Abu Bakr from Muhammad.  Jafar was a direct descendant of Abu Bakr from his maternal side, as well as being a descendant of Ali from his father's side. Jafar al-Sadiq was also the successor of the Naqshbandi Sufi order believed to be originating from Abu Bakr himself.  Imam Muhammad al Baqir, the father of Imam Jafar Sadiq, also called Abu Bakr with the title Siddiq.