Is it permissible for someone who has started the prayer and said the opening Takbir (Takbirat al-Ihram) to join a congregation that has already begun the prayer, or to join them after they have bowed (ruku')?

Scholars have differed on the ruling concerning a worshipper changing their intention from praying alone to praying behind an imam. The majority of scholars (Hanafis, Malikis, and Hanbalis) maintain that it is not permissible, because the intention to pray behind an imam must be formed at the beginning of the prayer. Meanwhile, the Shafi'is and one narration from Imam Ahmad held it permissible, citing its occurrence in the Prophetic Sunnah (definition), where it is established that a worshipper can transition from praying behind one imam to another, from praying alone to becoming an imam, and from praying behind an imam to praying alone. Consequently, it is permissible to change the intention from praying alone to praying behind an imam.

This latter opinion is the more preponderant one. It is permissible to make this transition whether it is at the beginning of the prayer or after performing one or more rak'ahs. If the worshipper completes their prayer before their imam, they should sit and not follow the imam, then make the taslim (definition) with him, or intend to pray alone and make the taslim.