What is the ruling when a man accuses another man with explicit insults related to incest?

Primary text

If a man says to another man, 'O groomer of your mother' (Ya nakih ummih), and the mother is alive, the accuser incurs the punishment for slander (hadd) against the accused man, and the mother is also entitled to a prescribed punishment (hadd) against the accuser. This ruling is reported from Imam Ahmad when asked about this specific phrase, and it is also supported by Muhanna.