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. "
English reference: Vol. 3, Book 11, Hadith 2127
Arabic reference: Book 11, Hadith 2208