Musannaf Ibn Abi Shayba 13:787

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Ibn Abi Layla, from al-Hakam; and from Mansur, from Ibrahim; and from Sufyan, from Jabir, from al-Sha’bi, from Shurayh; they both said: If he acknowledges a debt to an heir during his terminal illness, it is not valid except with evidence. But if he acknowledges it to a non-heir, it is valid.