Musannaf Ibn Abi Shayba 26:536

Waki’ narrated to us, from Abu ‘Asim, from al-Sha’bi, who said: Shurayh used to treat *al-wala’* (the right of inheritance through manumission) as he treated wealth. Al-Sha’bi said: And the people of Medina say: *Al-wala’* belongs to the elder.