Reference for Shevuot 40:1
אלא לרבא קשיא
<br> but according to Raba, it is difficult? - Raba may say to you, explain it thus: What is the binding force of a vow which is mentioned in the Torah? He who says: I take it upon me that I shall not eat meat, and that I shall not drink wine, as on the day that my father died, or, as on the day that So-and-So was killed; [he is prohibited from eating meat, etc.;] and Samuel said: only if he had already made a vow on that day. What is the reason Scripture says: If a man vow a vow unto the Lord - only if he vow in the matter which he had already vowed. - 'As on the day my father died'! This is self-evident? - 'As on the day that Gedaliah, son of Ahikam, was killed' is necessary. I might have thought that, since it is also prohibited even if he had not vowed, the fact that he vowed does not bring a prohibition upon him [because of his vow]; so that it [his present vow] is not based on a [previous] vow, [and hence is not a normal vow]; therefore he teaches us [that it is so based; and because perforce he mentions this clause, he mentions also the previous clause, though it is unnecessary]. And R. Johanan also holds this view of Raba, for when Rabin came [from Palestine] he said that R. Johanan said: [If one says:] 'Mibta that I shall not eat of thine', or, 'Issar that I shall not eat of thine', it is an oath. When R. Dimi came [from Palestine] he said that R. Johanan said: [If one says: T swear] I shall eat', or, '[I swear] I shall not eat', [and he transgresses the oath,] it is a false oath; and its prohibition is [derived] from this [verse]: Ye shall not swear by My name falsely. [If one says: T swear] I have eaten' or, '[I swear] I have not eaten', [and it was untrue,] it is a vain oath, and its prohibition is [derived] from this [verse]: Thou shalt not take the name of the Lord thy God in vain. Vows come under the prohibition of: He shall not break his word. <br>