Translation
(11) The Chapters on Expiation
كتاب الكفارات
(17) Chapter: One who makes a vow but does not state it specifically
(17) باب مَنْ نَذَرَ نَذْرًا وَلَمْ يُسَمِّهِ
Sunan Ibn Majah 2127 
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 ‏"‏ ‏.‏
Grade :  Hasan (Darussalam)
Reference : Sunan Ibn Majah 2127
In-book reference : Book 11, Hadith 38
English translation : Vol. 3, Book 11, Hadith 2127