Numbers 10:31

אל נא תעזב LEAVE [US] NOT, I PRAY THEE — The word נא is an expression of entreaty. Here it means: I beg of you, not to leave usin order that people should not say, “Jethro did not become a proselyte out of pure love for the Jewish faith. He believed that the proselytes, too, would have a portion in the Land; now, however, that he saw that they will have no portion in it he has deserted them and gone his own way (Sifrei Bamidbar 80). כי על כן ידעת FOR THOU KNOWEST OUR ENCAMPMENT IN THE WILDERNESS] — For (כי) it is fitting for you to do this, because (על כן) you have known our encampments in the wilderness and you have seen the miracles and the mighty deeds that have been wrought for us. כי על כן ידעת — This is similar to על אשר ידעת, like (Genesis 38:26) “For (כי) she has acted rightly, because (על כן) I did not give her to Selah, my son”; (Genesis 19:8): “For (כי) this kindness please do out of respect to me, because (על כן) they have come under the shadow of my roof”; (Genesis 33:10): “For (כי) it is fitting and proper for you to accept my present, because (על כן) I have seen your face." והיית לנו לעינים — The verb is in the past tense and we have to understand it just as the Targum renders it: and all the mighty deeds that have been wrought for us thou hast seen with thine own eyes. Another explanation is that it is the future tense: whatever things will be hidden from our eyes, you will enlighten our eyes about it. Still another explanation is that the passage means that you will be held in affection by us as our “very eye-balls”. as it is said, (Deuteronomy 10:19) “and ye shall love the stranger” (Sifrei Bamidbar 80).

וַיֹּ֕אמֶר אַל־נָ֖א תַּעֲזֹ֣ב אֹתָ֑נוּ כִּ֣י ׀ עַל־כֵּ֣ן יָדַ֗עְתָּ חֲנֹתֵ֙נוּ֙ בַּמִּדְבָּ֔ר וְהָיִ֥יתָ לָּ֖נוּ לְעֵינָֽיִם׃

And he said, Leave us not, I pray thee; since thou knowst how we are to encamp in the wilderness, and thou mayst be to us instead of eyes.