If one forgets a unit of prayer, does he repeat the prayer or just the unit?

I and another person entered during the midday prayer, and we were entering while the imam had already started the third unit. When he finished, we stood up to complete the two units. I completed the two units, but he did not complete anything other than the third unit, then he sat down. When I finished the closing salam, I wanted to inform him that he had missed a unit, but he joined the imam in the funeral prayer. When the funeral prayer ended, I informed him that he had missed a unit he had not prayed. In this case, does he repeat the prayer or just the unit?The Shaykh: The question says: A man entered with the imam during the midday prayer and missed two units. When the imam finished the closing salam, a funeral was brought in, and this man prayed one unit after the imam’s salam and feared that he would miss the funeral, so he returned to the rest of his prayer and joined the funeral prayer, then he wanted to make up one unit. The questioner: He was forgetful.

The Shaykh: Yes.
What I said was that he was forgetful.
This man, when he prayed one unit after the imam’s salam, forgot, said the closing salam, and joined the imam in the funeral prayer, then he completed his prayer after the funeral prayer.So we say: This does not make his prayer valid due to the interruption between the parts of the prayer. It is obligatory for him that when he remembered, he complete his prayer, even if he missed the funeral prayer; because his prayer is an obligatory individual obligation, and the funeral prayer is a collective obligation, and those who were sufficient have already performed it.