From Sahih Bukhari (Hadith 6698):
Narrated Sa`id bin 'Ubada Al-Ansari:that he consulted the Prophet (ﷺ) about a vow that had been made by his mother who died without fulfilling it. The Prophet (ﷺ) gave his verdict that he should fulfill it on her behalf. The verdict became Sunna (i.e. the Prophet's tradition)
Reference: Bukhari 6698