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).

أَخْبَرَهُ أَنَّ سَعْدَ بْنَ عُبَادَةَ الأَنْصَارِيَّ اسْتَفْتَى النَّبِيَّ ﷺ فِي نَذْرٍ كَانَ عَلَى أُمِّهِ فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ فَأَفْتَاهُ أَنْ يَقْضِيَهُ عَنْهَا فَكَانَتْ سُنَّةً بَعْدُ