If someone is praying a voluntary salah and hears the iqamah, what should he do?

Ibrahim says that if a person comes to the mosque and thinks that the people have already performed the prayer, so he makes the intention to perform the obligatory prayer, and after he has completed two rak'ahs, the iqamah for the prayer is given. Now he should join the imam in his prayer; when the imam has performed two rak'ahs (making his total four), he should say salam and remain with the imam for the rest of the prayer, and make the last two rak'ahs with the imam as nafl (supererogatory) prayers.

Hazrat Hammad also says the same.