The obligatory prayer behind the one praying a voluntary prayer

Q: What is your opinion, Your Eminence, regarding the obligatory prayer behind the one praying a voluntary prayer?

A: There is no harm in the obligatory prayer behind the one praying a voluntary prayer; because it is established from the Prophet (peace and blessings be upon him) in some types of the prayer of fear that he prayed two units with one group, then said the closing salam, then prayed two units with another group, then said the closing salam. So the first was obligatory for him and the second was voluntary, while the ones praying behind him were performing the obligatory prayer.
It is also established in the two Sahihs from Mu'adh ibn Jabal (may Allah be pleased with him) that he used to pray the night prayer with the Prophet (peace and blessings be upon him), then he would return to his people and pray that prayer with them. So it was voluntary for him and obligatory for them. Similarly, if a person is present in Ramadan and they are praying the tarawih, and he has not prayed the obligatory night prayer, he prays the night prayer with them to obtain the reward of the congregation. When the imam says the closing salam, he stands up and completes his prayer.