Hadith 514050
Chapter 36: Judgements - كتاب الأقضية
وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ لاَ تَجُوزُ شَهَادَةُ خَصْمٍ وَلاَ ظَنِينٍ .
Malik related to me that Umar ibn al-Khattab said, "The testimony of some one known to bear a grudge or to be unreliable is not accepted."
Reference Muwatta Malik 514050
USC-MSA web (English) reference : Book 36, Hadith 4