Sa'id bin Jubayr (may Allah have mercy on him) says that the matter of (the slanderer's) repentance, (to be saved) from the great punishment, is between Allah, the Lord of Might, and the servant, and his testimony will not be accepted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20575