Al-Matalib al-'Aliyah 2195 — Those whose testimony is not accepted

It is narrated from Talhah bin Abdullah bin Awf (may Allah have mercy on him) that the Prophet (peace and blessings be upon him) ordered a caller, and when he reached the mountain pass he called out: "The testimony of a party to a dispute (i.e. anyone who is a party to a dispute, whether claimant or defendant) and of a suspect person (one whose truthfulness or honesty is doubted) is not acceptable, and the oath is binding on the defendant."
Abu Dawud narrated it in his book al-Marasil.