Imam al-Shafi'i (may Allah have mercy on him) said: "Because he is among his forefathers, he is testifying in favour of something that is part of himself; and his sons are part of him, so it is as though he testified in favour of a part of himself."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / Q20862
Al-Miswar bin Makhramah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Fatimah is a part of me; whoever hurts her has hurt me."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20862
Sayyidah Khawlah bint Hakim (may Allah be pleased with her), the wife of Sayyiduna Uthman bin Maz'un (may Allah be pleased with him), said that the Prophet (peace and blessings be upon him) came out holding one of his (grandsons) to his chest, and he was saying: "By Allah, you make (people) cowardly, you make them ignorant, and you make them stingy; and indeed you are among the sweet basil of Allah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20863
Ibrahim bin Muhammad al-Ribati related in Rajab of the year 266 AH that it was read out to Abu Ubayd.
(b) Sayyidah Aishah (may Allah be pleased with her) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The testimony of a treacherous man and a treacherous woman, of one who harbours rancour against his brother, of one who makes accusations regarding clientage and kinship, and of a servant in favour of his master shall not be accepted."
This is the wording of Ali's narration; in al-Ribati's narration it is: "nor one under suspicion, nor one suspected on account of kinship." The first is more correct, and this Yazid is da'if (weak).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20864
Uqayl narrated from al-Zuhri (may Allah have mercy on him) that he said: "The Sunnah has been established that the testimony of a disputant or of one under suspicion is not permissible."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20865
Ibn Shihab (may Allah have mercy on him) said: The practice of Islam has been established that the testimony of a quarrelsome person and of an untrustworthy person shall not be accepted, and the testimony of a disputant shall not be accepted regarding his opponent.
The rejection of the testimony of one under suspicion has been narrated to us in general terms through two mursal routes from the Prophet (peace and blessings be upon him), and through another route as connected (mawsul), although there is weakness in it; it is strengthened by the two mursal routes along with it. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20866