What is the ruling of Islamic law regarding the right of a divorced mother to visit her three-year-old son, who is under her custody, in light of the father's refusal to enable her to do so, and what are the evidences from the Qur'an and Sunnah or from the actions of the Companions that confirm this right?

The mother has a greater right to custody of the child than the father when the parents separate, unless there is an impediment preventing her from doing so, or the child is discerning and chooses one of them. This is because the mother is more capable of caring for and raising the child, and she has patience and time that a man does not possess. It is also established in the Sunnah (definition) that the Prophet, peace and blessings be upon him, ruled that the mother has a greater right to her son unless she marries, and Abu Bakr al-Siddiq affirmed this.