How was the worship of the Prophet, peace and blessings be upon him, in the Cave of Hira before the advent of his prophethood, and are there reliable sources that can be referenced to detail this phase?

Before his prophethood, the Prophet, peace and blessings be upon him, used to seclude himself in the cave of Hira for worship, as narrated by Aisha, may Allah be pleased with her. He would "tahannath" there, meaning he would follow the Hanafiyyah, the religion of Ibrahim, peace be upon him. One can refer to books of biography such as "Al-Seerah Al-Nabawiyyah" by Ibn Hisham, "Al-Bidayah wa Al-Nihayah" by Ibn Kathir, and "Al-Raheeq Al-Makhtum" by Al-Mubarakpuri. This seclusion was a divine arrangement to prepare the Prophet for the great matter that awaited him.