Matthew 5:36

μήτε ἐν τῇ κεφαλῇ σου ὀμόσῃς ὅτι οὐ δύνασαι μίαν τρίχα λευκὴν ἢ μέλαιναν ποιῆσαι.

Neither shall you swear by your head, because you are not able to make one hair white or black.