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."
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