Explaining what a person praying alone should do when another congregation is established

Question:
A man was praying the obligatory prayer alone, and during that time, the people of the mosque entered and began the congregational prayer. Should he break his prayer, or should he change his intention to make it a voluntary prayer so that he can pray with them?

Answer:
It is better to change it to a voluntary prayer and then pray the congregational prayer with those who entered, in order to obtain the virtue of the congregation. If he breaks it and prays with them, there is no harm, because he is breaking it for a legitimate religious interest that benefits the prayer itself. And success is from Allah.