Abu Bakr refuted witnesses that were produced and their testimonies were refused on different grounds
We read in Futuh al Buldan, page 35:
Malik bin Jawuna narrates from his father that Fatima said to Abu Bakr: ‘Rasulullah (s) bestowed Fadak to me, so return it.’ Ali testified in her favour, Abu Bakr asked for another witness, and Umm Ayman testified in support of Fatima. Upon this, Abu Bakr said: ‘O daughter of Rasulullah (s), such testimony is unacceptable unless you have two males or one male and two females, upon hearing this Fatima left.’
We also read:
Jafar bin Muhammad said that Fatima said to Abu Bakr: ‘Return Fadak to me as it was given to me by Rasulullah (s)’. Abu Bakr demanded witnesses. Umm Ayman and Rabah the Servant of Rasulullah (s) testified in support of her claim. He said that such testimonies were unacceptable unless presented by one man and two women.
We also read:
Umm Hani states that Fatima daughter of Rasulullah (s) appeared before the Court of Abu Bakr and asked: ‘When you die who shall inherit you?’ He replied: ‘My family and descendants’. Fatima said: ‘What is your stance when you seize the inheritance of the Prophet (s) and do not give it to us?’ He said: ‘Oh daughter of Allah’s messenger, I did not seize your father’s gold and silver and this or that.’ Fatima then said: ‘Give us our share of Khayber, Fadak is our exclusive property’. He [Abu Bakr] replied: ‘Oh daughter of Rasulullah (s), I heard the Prophet (s) say that (Fadak) is a pillar that Allah (swt) has provided for me as a source of earnings during my life time, verily when I die, distribute this amongst the Muslims!.’
We read in Wafa al Wafa, Volume 3 page 999 Bab Sadaqat un Nabi:
“Fadak was that land that Hadhrat Fatima claimed Rasulullah (s) had given to me. Abu Bakr demanded witnesses. ‘Ali and Umm Ayman testified. The Khaleefa replied ‘Only the testimony of a man and two women are acceptable”.