What is the ruling on someone who was praying alone and then completed his prayer with a congregation he found? And is that permissible?

If a person performs ihram individually and then intends to switch to congregational prayer, becoming a ma'mum (follower), and completes his prayer with them, there is a difference of opinion among scholars regarding the validity of his prayer. Many scholars have affirmed the validity of this prayer, but repeating it is more cautious to avoid the اختلاف (disagreement).