Sahih Bukhari
Oaths and Vows
Volume: 008 / Book Number: 078 / Hadith Number: 689
Narrated Sa'id bin 'Ubada Al-Ansari:
that he consulted the Prophet about a vow that had been made by his mother who died without fulfilling it. The Prophet gave his verdict that he should fulfill it on her behalf. The verdict became Sunna (i.e. the Prophet's tradition).