33 “Again, you have heard that it was said to an older generation, ‘Do not break an oath, but fulfill your vows to the Lord.’ 34 But I say to you, do not take oaths at all—not by heaven, because it is the throne of God, 35 not by earth because it is his footstool, and not by Jerusalem, because it is the city of the great King. 36 Do not take an oath by your head because you are not able to make one hair white or black. 37 Let your word be ‘Yes, yes’ or ‘No, no.’ More than this is from the evil one.
The Gemara raises an objection: Rabbi Yosei, son of Rabbi Yehuda, says: What is the meaning when the verse states: “A just ephah, and a just hin, shall you have” (Leviticus 19:36)? But wasn’t a hin included in an ephah? Why is it necessary to state both? Rather, this is an allusion that serves to say to you that your yes [hen] should be just, and your no should be just. Apparently, it is a mitzva for one to fulfill his promises. Abaye says: That verse means that one should not say one matter with his mouth and think one other matter in his heart. It is prohibited for one to make a commitment that he has no intention of fulfilling. Rav Kahana made his commitment in good faith and reneged due to changed circumstances. That is not prohibited.