Hadith 13:13
Abu Dawud has from the narration of Ibn Abbas (RA) (who reported Allah's Messenger (ﷺ) 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 which he is unable to fulfill , its atonement is the same as that for an oath." [Its chain of narrators is authentic, but the Hadith scholars held that the strongest view is that it is Mawquf (saying of a Companion)].
وَلِأَبِي دَاوُدَ: مِنْ حَدِيثِ اِبْنِ عَبَّاسٍ مَرْفُوعاً: { "مِنْ نَذَرَ نَذْراً لَمْ يُسَمِّهِ, فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ, وَمَنْ نَذَرَ نَذْراً فِي مَعْصِيَةٍ, فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ, وَمَنْ نَذَرَ نَذْراً لَا يُطِيقُهُ, فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ" } وَإِسْنَادُهُ صَحِيحٌ; إِلَّا أَنَّ اَلْحُفَّاظَ رَجَّحُوا وَقْفَهُ
Original Reference: Bulugh al-Maram 1386