Abu Ishaq al-Sabay'ai' أبو إسحاق السبيعي عمرو بن عبد الله · The Chapters on Expiation

"Whoever makes a vow and does not state it specifically, the expiation (for such a vow) is the expiation for breaking an oath. Whoever makes a vow and is not able to fulfill it, the expiation for that is the expiation for breaking an oath. Whoever makes a vow and is able to fulfill it, let him do so."