What is the ruling on vowing continuous fasting (for a month or four months) with the intention of disciplining oneself, coupled with the condition, "If I believe in Allah the Almighty and the Last Day," and does this condition make it similar to saying, "He is a Jew or a Christian if he does not do such and such"?

A vow of obstinacy (nadhr al-lajāj) gives the one who made the vow a choice between fulfilling the vow or paying an expiation for an oath (kaffārat yamīn). As for a person's statement: "If I truly believe in Allah the Almighty and the Last Day, I will fast for a whole month if I do such-and-such," this is an encouragement for the self to adhere [to a commitment], and is not like saying: "He is a Jew or a Christian if he does not do such-and-such." This latter statement is forbidden. Scholars have differed regarding it: does it necessitate disbelief if the swearer venerates that by which he swore, or only expiation? The preponderant opinion is that whoever says that and then breaks their promise must offer an expiation for an oath.