The Prophet ﷺ said: If anyone takes a vow but does not name it, its atonement is the same as that for an oath, if anyone takes a vow to do an act of disobedience, its atonement is the same as that for an oath, if anyone takes a vow he is unable to fulfill, its atonement is the same as that for an oath, but if anyone takes a vow he is able to fulfill, he must do so. Abu Dawud said: This tradition has been transmitted by Waki' and others on the authority of 'Abd Allah b. Sa'id b. Abi al-Hind, but they traced it no farther back than Ibn 'Abbas.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ مَنْ نَذَرَ نَذْرًا لَمْ يُسَمِّهِ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ وَمَنْ نَذَرَ نَذْرًا فِي مَعْصِيَةٍ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ وَمَنْ نَذَرَ نَذْرًا لاَ يُطِيقُهُ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ وَمَنْ نَذَرَ نَذْرًا أَطَاقَهُ فَلْيَفِ بِهِ