The Messenger of Allah ﷺ said, "Whoever makes a vow without specifying its content, its expiation is the expiation of an oath."
حدثنا علي بن محمد حدثنا وكيع حدثنا إسمعيل بن رافع عن خالد بن يزيد
قال رسول الله ﷺ من نذر نذرا ولم يسمه فكفارته كفارة يمين
[Hasan] * Ismail bin Rafi, previously mentioned, no. 1337, and his hadith has a hasan corroborating witness, see the following hadith.