ذٰلِكَ أَدنىٰ أَن يَأتوا بِالشَّهادَةِ عَلىٰ وَجهِها أَو يَخافوا أَن تُرَدَّ أَيمانٌ بَعدَ أَيمانِهِم ۗ وَاتَّقُوا اللَّهَ وَاسمَعوا ۗ وَاللَّهُ لا يَهدِي القَومَ الفاسِقينَ

This is more likely to ensure that they will give truthful testimony, or else their oaths may be refuted. So, fear God and listen carefully. God doesn’t guide the sinful.