Rashi on Numbers 21:14/רש"י על במדבר כ״א:י״ד

1על כן יאמר WHEREFORE IT IS SAID (more lit., about this it will be said) — About this encampment and the miracles that were wrought at it, it will be told בספר מלחמות ה׳ IN THE NARRATIVE (lit., THE BOOK) OF THE WARS OF THE LORD — whenever people narrate the miracles that were wrought for our fathers they will declare את והב.

2את והב — is equivalent to את יהב; just as from the root יעד one may say וָעֵד, so from יהב, "to give” one may say והב: the ו is a root-letter. It means to say: they will relate what (את) He gave (והב) to them (i.e. what boons He gave to them) and how He did many miracles at the Red Sea (בים סוף being the equivalent of בסופה in the text).

3ואת הנחלים ארנון AND THE VALLEYS AT ARNON — Just as they relate the miracles at the Red Sea, so there should be related, too, the miracles at the valleys of Arnon, for here, also, great miracles were wrought (Midrash Tanchuma, Chukat 20). And what were those miracles? (the reply is given in the next passage).