Grading: Its chain is da'if (weak).
Technical note / Defect ('illah): The author's shaykh "Abd al-Rahman bin al-Hasan" is "layyin" (weak), and Muslim bin Khalid (al-Makhzumi, known as al-Zanji) is also a weak narrator.

Reference / Source: Imam al-Bayhaqi narrated it (10/201) by way of Ubaydullah bin Musa and Abd al-Samad bin al-Nu'man; both of them transmit it from Muslim bin Khalid al-Zanji with this same chain.
Details of the narration: In al-Bayhaqi, the first wording is: "The testimony of a friend, an insane person, or a spiteful enemy is not permissible." The second wording is: "The testimony of a spiteful enemy and a suspected person is not permissible." Imam al-Bayhaqi said that the word "al-zinnah" (suspicion) is better preserved (more correct) than "al-khullah" (friendship).

Note / Clarification: Then Imam al-Bayhaqi transmitted its explanation from one of the narrators, saying: "al-jinnah" means insanity, and "al-hinah" refers to someone between whom and you there is enmity.

Important point: Ibn al-Athir wrote in "al-Nihayah" that "al-hinah" means enmity, and it is a rarely used variant of "al-ihnah".

Reference / Source: Imam Abu Dawud narrated it in "al-Marasil" (397), and al-Bayhaqi (10/201), by way of Ibn Abi Dhi'b, from al-Hakam bin Muslim, from Abd al-Rahman al-A'raj as "mursal", that the Messenger of Allah (peace and blessings be upon him) said: "The testimony of a suspected person, a spiteful enemy, and an insane person is not permissible."

Grading: Imam al-Bayhaqi indicated that it is the most authentic narration in this chapter, even though it is "mursal".
Technical note / Defect ('illah): We say: two narrators reported from al-Hakam bin Muslim, and Imam Ibn Hibban mentioned him in "al-Thiqat".

Technical note / Defect ('illah): Abd al-Razzaq narrated it (15366) by way of Ibn Abi Dhi'b, from al-Hakam, but here Abd al-Rahman is taken to be "Ibn Farrukh" and not "Abd al-Rahman al-A'raj".

Reference / Source: Abd al-Razzaq narrated (15365) by way of Ibrahim bin Muhammad al-Aslami, from Abdullah, from Yazid bin Talhah, from Talhah bin Abdullah, from Abu Hurayrah (may Allah be pleased with him) as marfu': "The testimony of an opposing litigant and a suspected person is not permissible."

Grading: Ibrahim al-Aslami is a "matruk" (abandoned) narrator, and his making it "mawsul" is munkar;
Technical note / Defect ('illah): The correct view is that this narration is "mursal", as it is in "Marasil Abi Dawud" (396). For more hadiths of this chapter, see the hadith of Abdullah bin Amr in "Musnad Ahmad" (11/6698).