What is the ruling on my prayer and what ensues from it in terms of prostration of forgetfulness (sujud al-sahw) and repetition, after I prayed with a congregation that erred in the number of rak'ahs, then I greeted (salāmed) and rejoined them again to complete it?

If the Imam makes the Taslim before completing his prayer and then returns to complete it, the latecomer (masbuq) is obligated to follow the Imam and perform the prostrations of forgetfulness (sujud al-sahw) with him, and then complete his own prayer. If the latecomer intentionally makes the Taslim before completing his prayer, his prayer is invalid and must be repeated. However, if he makes the Taslim out of forgetfulness, his prayer is not invalidated, and he should perform the prostrations of forgetfulness at the end of his prayer.