It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Sayyiduna Sa'd bin Ubadah (may Allah be pleased with him) said: O Messenger of Allah! My mother has died, and a vow was due from her; will it suffice if I free a slave on her behalf? He (peace and blessings be upon him) said: "Free a slave on behalf of your mother."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5349