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  

[Machine] She fasted for a month, and then Allah saved her, so she did not fast until she died. Then her daughter or sister came to the Messenger of Allahﷺ and he ordered her to fast on her behalf. The rest of the narrations about it have been mentioned in the book of fasting and the book of Hajj. And with Allah is the success.  

تَصُومَ شَهْرًا فَنَجَّاهَا اللهُ فَلَمْ تَصُمْ حَتَّى مَاتَتْ فَجَاءَتْ بِنْتُهَا أَوْ أُخْتُهَا إِلَى رَسُولِ اللهِ ﷺ فَأَمَرَهَا أَنْ تَصُومَ عَنْهَا سَائِرُ الرِّوَايَاتِ فِيهِ قَدْ مَضَتْ فِي كِتَابِ الصِّيَامِ وَكِتَابِ الْحَجِّ وَبِاللهِ التَّوْفِيقِ