A woman came to the Messenger of Allah (peace and blessings be upon him) and said that her mother had vowed to perform Hajj but died before performing it; should she perform Hajj on her behalf? He (peace and blessings be upon him) said: "Yes! Perform Hajj on her behalf. What do you think: if your mother owed a debt, would you pay it?" She said: Yes! He (peace and blessings be upon him) said: "Then pay Allah as well, for He is the most deserving of fulfilment."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8672