[M]If a husband accuses his wife of zina without 4 witnesses, he must perform li'an. (Qur'an 24:6-9)
[M]The husband swears 4 times by Allah that he is truthful, then a 5th invoking Allah's curse.
[M]The wife swears 4 times he is lying, then a 5th invoking Allah's wrath.
[1]Know that li'an results in permanent separation with no possibility of remarriage.
[2]Know that the child born after li'an is attributed to the mother alone.