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)]. (DE)
Reference (DE): Bulugh al-Maram 1386 (DE)
In-book reference (DE): Book 13, Hadith 13 (DE)
English translation (DE): Book 13, Hadith 1386 (DE)