Sahih al-Bukhari 6698Book 83, Hadith 75
حَدَّثَنَا أَبُو الْيَمَانِ، أَخْبَرَنَا شُعَيْبٌ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ، أَنَّ عَبْدَ اللَّهِ بْنَ عَبَّاسٍ، أَخْبَرَهُ أَنَّ سَعْدَ بْنَ عُبَادَةَ الأَنْصَارِيَّ اسْتَفْتَى النَّبِيَّ صلى الله عليه وسلم فِي نَذْرٍ كَانَ عَلَى أُمِّهِ، فَتُوُفِّيَتْ قَبْلَ أَنْ تَقْضِيَهُ. فَأَفْتَاهُ أَنْ يَقْضِيَهُ عَنْهَا، فَكَانَتْ سُنَّةً بَعْدُ.
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).