Oaths and Vows

Bukhari :: Book 8 :: Volume 78 :: Hadith 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).