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). (Using translation from Bukhārī 6698)
إِنَّ سَعْدَ بْنَ عُبَادَةَ الْأَنْصَارِيَّ ؓ اسْتَفْتَى رَسُولَ اللهِ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ وَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَأَمَرَهُ رَسُولُ اللهِ ﷺ أَنْ يَقْضِيَهُ عَنْهَا فَكَانَتْ سُنَّةً بَعْدُ