The Prophet (peace and blessings be upon him) said:
"If nothing (specific) is named in a vow, its expiation is (the same as) the expiation for an oath."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4488
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[حم م ٣] عن عقبة بن عامر. الإرواء ٢٥٨٦.