The Ruling on the Condition of Intention in Leading the Prayer

Q: If I enter the mosque after the congregation has finished the prayer, and I establish the prayer and say the takbir of ihram, and then a man comes after me and enters into my prayer with me, while I did not intend to lead, is his prayer valid or not?

A: The correct view is that it is prescribed for you to intend to lead the prayer when one or more people enter the prayer with you; because the congregation is sought after and it has great virtue. Some scholars have held that this is only valid in voluntary prayers, but the correct view is that it is valid in both obligatory and voluntary prayers; because the original rule is that they are equal in rulings except for what is specified by evidence. It is established that the Prophet (peace and blessings be upon him) used to pray at night alone in the house of Maymunah, the aunt of Ibn Abbas (may Allah be pleased with them both). Ibn Abbas stood up, performed wudu, and lined up to the left of the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) moved him to his right and prayed with him. This is agreed upon by al-Bukhari and Muslim.
Muslim narrated in his Sahih from the Prophet (peace and blessings be upon him) that he was praying alone, so Jabir and Jubayr came and lined up to his right and left. He placed them behind him and prayed with them. These two hadiths indicate what we have mentioned, as they also indicate that one person stands to the right of the imam, and two or more stand behind him.