What is the ruling on my prayer after interrupting it with the intention of re-performing it alone, given that another congregation was praying but I did not join them?

If a person begins a mandatory prayer alone and then the congregational prayer is established, they have the option to either interrupt their prayer and join the congregation, or change their intention to a supererogatory (nafilah) prayer, complete it, and then join the congregation.

If they fear missing the congregational prayer, it is better to interrupt their prayer immediately and join the congregation. This interruption is not forbidden if it is for the purpose of moving to something better.

Interrupting a mandatory prayer after starting it without a valid excuse is forbidden, just as not joining the congregation is impermissible. This is because congregational prayer is, according to the preponderant opinion, an individual obligation (wajib 'ala al-ayan). However, the prayer is valid without it if it fulfills all its conditions and pillars.