مُوَطَّأُ مَالِكٍ
Muwatta Malik 1406
Judgements
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."
Arabic/English book reference: Book 36, Hadith 4