Imam Sadiq (a.s.) informs us why Ameerul Momineen (a.s.) did not take back Fadak on assuming caliphate.

He narrates: Ali (a.s.) did not take it back because the oppressor and the oppressed (Hazrat Fatima Zahra s.a.) had already set forth towards Allah, the Almighty. Allah had rewarded the oppressed (for patience) and had punished the oppressor (for usurping). Then Ali (a.s.) did not wish to take back something for which Allah had already punished the oppressor and had rewarded the oppressed.

  • Elal al-Sharae p 155 under Chapter of ‘Why Ameerul Momineen (a.s.) left Fadak as it is on assuming caliphate’
  • Awalim al-Uloom v 11 p 766