Hazrat Tawus says that if a person comes to the mosque and performs his obligatory prayer alone, and then a congregation is established, he should perform the prayer with the congregation and should not count his previous prayer.

Hazrat Sha'bi, Hasan, Hazrat Sha'bah, and Hazrat Hakam state that he should complete his prayer with salam and then join the prayer with the imam.

4929 - Yunus and Mansur narrated to us from al-Hasan.

4930 - And Hajjaj from Ata, and Shu’bah from Al-Hakam said: He gives salam (then) (1) joins the imam in his prayer.

Hazrat Abdullah bin Utbah says that one should break his own prayer and join their congregation.

Hazrat Hammad says that I like that he speaks and then joins their group.

Hazrat Sha'bi says that if a person starts an obligatory prayer and the iqamah is called for that same prayer, then he should break his prayer; it will become a voluntary (nafl) prayer for him, and he should join them in the obligatory prayer.