Rashi on Exodus 17:15/רש"י על שמות י״ז:ט״ו

1ויקרא שמו AND HE CALLED HIS (or its) NAME — the name of the altar,

2ה׳ נסי (lit., the Lord is my banner or my miracle) — The Holy One, blessed be He, here performed a miracle for us. It does not mean that the altar was named ה׳, “the Lord, my banner”, but the reason for so calling it was that anyone mentioning the altar’s name would thereby remember the miracle which the Omni-present had performed, since he would be saying: the Lord, He is our Miracle (cf. Rashi on Genesis 33:20).