It is narrated from Sa'id bin al-Musayyib (may Allah have mercy on him) that when 'Umar bin al-Khattab (may Allah be pleased with him) enforced the hadd of qadhf on three men, he made them repent. Two of them repented, so he accepted their testimony, but Abu Bakrah (may Allah be pleased with him) refused to repent, so his testimony was also rejected.
Hadith Referenceمسند الإمام الشافعي / كتاب القضاء والأحكام والدعاوى والبينات واليمين ومع الشاهد والأيمان والشهادات / 1705
Hadith Takhrijاسناده ضعيف لابهام شيخ الشافعي : اخرجه البيهقي : 10 152 - وفى المعرفة السنن والآثار له (5892)۔