Sahih Bukhari
Oaths and Vows
Volume: 008 / Book Number: 078 / Hadith Number: 690
Narrated Ibn 'Abbas:
A man came to the Prophet and said to him, "My sister vowed to perform the Hajj, but she died (before fulfilling it)." The Prophet said, "Would you not have paid her debts if she had any?" The man said, "Yes." The Prophet said, "So pay Allah's Rights, as He is more entitled to receive His rights."