What is the ruling on a son owning his father’s land that he cultivated, after the father promised him ownership if he cultivated it, and some of the siblings objected to that?

It is not permissible for a father to single out one of his children with a gift over the others, unless there is a justification such as need. Otherwise, it is an injustice that engenders hatred. Justice must be observed among children, as the Prophet, peace and blessings be upon him, said to Bishr ibn Sa’d when he granted his son Nu’man a gift: “Have you given all your children a similar gift?” He said: “No.” The Prophet said: “Then do not ask me to bear witness to it, for I do not bear witness to injustice.” In another narration: “Fear Allah and be just among your children.” However, if the preference is for a valid reason or with the consent of the children, it is permissible. As long as the situation is as described, it is not permissible for the father to enforce the gift.