SHAFAQNA – The Grand Ayatollah Sistani answered a question about taking a vow (Nazr).
Question: Must a special saying be mentioned when taking a vow?
The Grand Ayatollah Sistani: Yes, a special saying must be mentioned when taking a vow; it is not necessary to say it in Arabic; so if it is said (for example), if my patient is cured, I undertake to pay 10 Tomans (a currency) to the poor in the way of God, this vow is correct. And if it is said I undertake to do such a thing for the sake of God, according to Wajib precaution, it (the undertaking) must be carried out, but if the name of God is not mentioned, and it is only said I take the vow, or the names of one of God’s saints is mentioned, the vow is not correct. And if the vow is correct and the person who undertakes it, does not carry it out on purpose, he/she has committed sin, and must pay Kaffarah (religious fine), and paying for Kaffarah for failing to fulfill the vow is the same as for breaking an oath.
Source: PERSIAN SHAFAQNA