It is narrated from Ibn Abbas (may Allah be pleased with him) that Sa'd bin Ubadah (may Allah be pleased with him) sought a ruling from the Messenger of Allah (peace and blessings be upon him) concerning a vow of his mother, who had died before fulfilling it. He (peace and blessings be upon him) said: "Fulfil the vow on her behalf."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوصايا / 12633