It was narrated from 'Uqbah bin 'Amit Al-Juhani that the Messenger of Allah (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. "
حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، حَدَّثَنَا إِسْمَاعِيلُ بْنُ رَافِعٍ، عَنْ خَالِدِ بْنِ يَزِيدَ، عَنْ عُقْبَةَ بْنِ عَامِرٍ الْجُهَنِيِّ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ " مَنْ نَذَرَ نَذْرًا وَلَمْ يُسَمِّهِ فَكَفَّارَتُهُ كَفَّارَةُ يَمِينٍ " .
IbnMaja
The Chapters on Expiation • Hadith 2127