Rashi on Genesis 16:9/רש"י על בראשית ט״ז:ט׳

1'ויאמר לה מלאך ה' וגו AND THE ANGEL OF THE LORD SAID UNTO HER etc. (see also Genesis 16:10 & Genesis 16:11). For each statement (in these three verses) a different angel was sent to her, and this explains why in reference to each statement the phrase is used, “and an angel said to her” (Genesis Rabbah 45:7).