Saeed bin Jubair (may Allah have mercy on him) says that I asked Sayyiduna Ibn Umar (may Allah be pleased with them both) about the issue of those who perform li'an. He replied that the Noble Prophet (peace and blessings be upon him) separated such a husband and wife and said: "Allah knows that one of you is certainly lying, so is any one of you ready to repent?" But neither of them was willing. The Noble Prophet (peace and blessings be upon him) repeated this statement before them three times, and upon their refusal, he separated them.