What was the first consensus (Ijma') in the history of Islam, and what is the significance of consensus?

Ijma' (consensus) is an important legislative source. It is the agreement of the qualified mujtahid scholars from the Muslim community, after the death of the Prophet, peace be upon him, on a particular matter. The first legitimate ijma' that occurred after the death of the Prophet, peace be upon him, was the consensus of the Companions on the necessity of appointing a caliph, and they all agreed on the leadership of Abu Bakr Al-Siddiq, may Allah be pleased with him. Among the early consensuses was also the Companions' agreement to fight the apostates. Ali ibn Abi Talib, may Allah be pleased with him, pledged allegiance to Abu Bakr Al-Siddiq on the day of Saqifah, and he had another confirmed pledge of allegiance after the death of Fatimah, may Allah be pleased with her.