lt was narrated from Ibn 'Abbas that the Prophet (ﷺ) said: "Whoever makes a vow and does not state it specifically, the expiation (for such a vow) is the expiation for breaking an oath. Whoever makes a vow and is not able to fulfill it, the expiation for that is the expiation for breaking an oath. Whoever makes a vow and is able to fulfill it, let him do so."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا عَبْدُ الْمَلِكِ بْنُ مُحَمَّدٍ الصَّنْعَانِيُّ، حَدَّثَنَا خَارِجَةُ بْنُ مُصْعَبٍ، عَنْ بُكَيْرِ بْنِ عَبْدِ اللَّهِ بْنِ الأَشَجِّ، عَنْ كُرَيْبٍ، عَنِ ابْنِ عَبَّاسٍ، عَنِ النَّبِيِّ ـ صلى الله عليه وسلم ـ قَالَ " مَنْ نَذَرَ نَذْرًا وَلَمْ يُسَمِّهِ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ وَمَنْ نَذَرَ نَذْرًا لَمْ يُطِقْهُ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ وَمَنْ نَذَرَ نَذْرًا أَطَاقَهُ فَلْيَفِ بِهِ " .
sunnah.com