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. 37Let your word be ‘Yes, yes’ or ‘No, no.’ More than this is from the evil one.38 “You have heard that it was said, ‘An eye for an eye and a tooth for a tooth.’ 39 But I say to you, do not resist the evildoer. But whoever strikes you on the right cheek, turn the other to him as well.
Date: 70-90 C.E. (based on scholarly estimates)
§ Rabbi Elazar says: No, or any negative expression, can be an oath, and yes, or any positive expression, can be an oath. The Gemara notes: Granted that no can be an oath, as it is written: “And the waters shall no more become a flood” (Genesis 9:15). And it is written with regard to that negative commitment: “As this is as the waters of Noah unto Me; as I have taken an oath that the waters of Noah shall no more pass over the earth” (Isaiah 54:9). But from where do we derive the fact that yes can be an oath? The Gemara answers: It is based on logical reasoning; from the fact that no can be an oath, yes too can be an oath. Rava said: And a negative expression is an oath only in a case where one said no, no, stating the term two times, or it is in a case where one said yes, yes, stating the term two times, as it is written: “All flesh shall not be excised any more by floodwaters” (Genesis 9:11), and it is again written: “And the waters shall no more become a flood” (Genesis 9:15). And from the fact that no is an oath only when stated two times, yes, too, is an oath only when stated two times.