Excerpt from 'The Life of Fatimah Az-Zahra', The Principle of All Women: Study and Analysis':
And from the severe procedures that Abu Bakr followed against Fatimah (s.a.) was his confiscating of Fadak to the public treasury.
Fadak was a village in Hijaz about two or three days (of travel) from Medina.
As for its boundaries to the Infallible Imams, Fadak comprehended all the regions of the Muslim world. Historians have mentioned that once Harun ar-Rasheed (the Abbasid caliph) said to Imam Musa bin Ja’far al-Kadhim (a.s.), 'I like to give Fadak back to you.'
Imam Musa al-Kadhim (a.s.) said, 'I do not take it except with all its limits.'
The caliph said, 'What are its limits?'
The Imam said, 'If I define it, you may not give it back.'
The caliph said, 'I adjure you by your grandfather to do.'
The Imam said, 'Its first limit is Aden.' The caliph changed color.
The Imam added, 'The second limit is Samarqand.' The caliph was astonished.
The Imam added, 'The third limit is Africa, and the fourth is after the islands and Armenia.'
Harun ar-Rasheed lost his mind and he said, 'He did not leave anything for us.'
The Muslim state that had covered most of the world was for the imams of the Ahlul Bayt (a.s.) but it was taken from them by force and the Umayyad and the Abbasid kings overcame it with no right.
Fadak was not conquered by the Muslim armies, but it was from that which Allah had granted to His prophet by peacemaking in the seventh year of hijra; therefore, it was completely the Prophet’s property.