Al-Awza'i (may Allah have mercy on him) said: I asked al-Zuhri (may Allah have mercy on him) about a man who testified before the ruler, and the ruler upheld his testimony, but later he changed his testimony.
It was asked: Is the first testimony valid or the later one? He said: Neither is acceptable.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 21195