The Messenger of Allah ﷺ said, "Whoever makes a vow without specifying it, its expiation is the expiation of an oath. Whoever makes a vow to commit a sin, its expiation is the expiation of an oath. Whoever makes a vow he is unable to fulfill, its expiation is the expiation of an oath. And whoever makes a vow he is able to fulfill, let him fulfill it."
حدثنا جعفر بن مسافر التنيسي عن ابن أبي فديك قال حدثني طلحة بن يحيى الأنصاري عن عبد الله بن سعيد بن أبي هند عن بكير بن عبد الله بن الأشج عن كريب
أن رسول الله ﷺ قال من نذر نذرا لم يسمه فكفارته كفارة يمين ومن نذر نذرا في معصية فكفارته كفارة يمين ومن نذر نذرا لا يطيقه فكفارته كفارة يمين ومن نذر نذرا أطاقه فليف به
Chain-checking, its isnad is hasan. Bayhaqi narrated it, 10/45, from the hadith of Abu Dawud through him, and Ibn Majah also narrated it, no. 2128. Talha bin Yahya's hadith is hasan, and Ibn Jurayj corroborated him, as found in Bayhaqi, 10/72. The hadith has supporting narrations.