The Messenger of Allah ﷺ said, "The expiation for an unspecified vow is the same as the expiation for breaking an oath."
حدثنا أحمد بن منيع حدثنا أبو بكر بن عياش حدثني محمد مولى المغيرة بن شعبة حدثني كعب بن علقمة عن أبي الخير
قال رسول الله ﷺ كفارة النذر إذا لم يسم كفارة يمين
No references are available.
Sourcing. Its chain is daif. Abu Dawud narrated it in the Book of Oaths and Vows, chapter on one who makes a vow without specifying it, no. 3323, from the hadith of Abu Bakr bin Ayyash with this wording, and he is daif and his teacher is unknown. It has a corroborating narration in Muslim, no. 1645, without the words "if he does not specify." With the corroborations, the hadith is sahih. This is the wording of Abu Dawud's hadith.