Ibn ‘Abbas reported God’s Messenger as saying, “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 fulfil, its atonement is the same as that for an oath; but if anyone takes a vow he is able to fulfil, he must do so.” Abu Dawud and Ibn Majah transmitted it, but some traced it no farther back than Ibn ‘Abbas.
Source: sunnah.com
Other translations
Arabic
وَعَنِ ابْنِ عَبَّاسٍ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «مَنْ نَذَرَ نَذْرًا لم يسمه فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ. وَمَنْ نَذَرَ نَذْرًا لَا يُطِيقُهُ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ. وَمَنْ نَذَرَ نَذْرًا أَطَاقَهُ فَلْيَفِ بِهِ» . رَوَاهُ أَبُو دَاوُدَ وَابْنُ مَاجَه وَوَقفه بَعضهم على ابْن عَبَّاس
sunnah.com