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