Imam al-Shafi'i (may Allah have mercy on him) said: "Since Allah Almighty mentioned li'an for husbands without qualification, li'an applies to every husband whose divorce is valid and upon whom the obligations are binding, and likewise to every wife upon whom the obligations are binding."
Ikrimah narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that Hilal bin Umayyah (may Allah be pleased with him) came, and he mentioned the long story of li'an; at its end it says that the Messenger of Allah (peace and blessings be upon him) said: "Had the oaths not already taken place, I would have dealt with this woman." Imam Ahmad (may Allah have mercy on him) says: Li'an is also called an oath.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللعان / 15293