Muwatta Malik
Judgements
Hadith #5
36.5
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