Sahih Bukhari Volume 3, Book 29, Hadith Number 77.

Narated By Ibn ‘Abbas : A woman from the tribe of Juhaina came to the Prophet and said, “My mother had vowed to perform Hajj but she died before performing it. May I perform Hajj on my mother’s behalf?” The Prophet replied, “Perform Hajj on her behalf. Had there been a debt on your mother, would you have paid it or not? So, pay Allah’s debt as He has more right to be paid.”