Muwatta Imam Malik Book of Setting Free and Wala', Hadith 24
Malik related to me that he had heard that Said ibn al-Musayyab spoke about a man who died and left three sons and left mawali whom he had freed. Then two of his sons died and left children. He said, "The third remaining son inherits the mawali. When he dies, his children and the children of his brothers share equally in the wala' of the mawali."
وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ أَنَّ سَعِيدَ بْنَ الْمُسَيَّبِ، قَالَ فِي رَجُلٍ هَلَكَ وَتَرَكَ بَنِينَ لَهُ ثَلاَثَةً وَتَرَكَ مَوَالِيَ أَعْتَقَهُمْ هُوَ عَتَاقَةً ثُمَّ إِنَّ الرَّجُلَيْنِ مِنْ بَنِيهِ هَلَكَا وَتَرَكَا أَوْلاَدًا . فَقَالَ سَعِيدُ بْنُ الْمُسَيَّبِ يَرِثُ الْمَوَالِيَ الْبَاقِي مِنَ الثَّلاَثَةِ فَإِذَا هَلَكَ هُوَ فَوَلَدُهُ وَوَلَدُ إِخْوَتِهِ فِي وَلاَءِ الْمَوَالِي شَرَعٌ سَوَاءٌ .