Rashi on Genesis 18:20/רש"י על בראשית י״ח:כ׳

1'ויאמר ה AND THE LORD SAID to Abraham, thus doing what he had said — that He would not conceal the matter from him.

2כי רבה BECAUSE IT IS GREAT — Wherever רבה elsewhere occurs in Scripture the accent is on the last syllable — on the ב — because it is to be translated by “great” (adj.) or “becoming great” (participle), but this, here, has the accent on the first syllable — on the ר — because it is to be translated by “has already become great” (perf.), just as I have explained (15:17) “And it came to pass, that, when the sun had gone down (בָּאָה)” and (Ruth 1:15) “Behold, thy sister-in-law has gone back (שָׁבָה)”.