Rashi on Exodus 11:2/רש"י על שמות י״א:ב׳

1דבר נא SPEAK נא — The word נא is always an expression of entreaty. Here it means: I entreat you, admonish them about this which follows (impress this injunction upon them), so that righteous man, Abraham, may not say: the prophecy (Genesis 15:13) “they shall serve them, and they shall afflict them” He permitted to be fulfilled in them, but the promise (Genesis 15:14) “and afterwards they shall go forth with great substance” He did not bring to fulfilment for them (Berakhot 9a-b).