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