Rashi on Exodus 4:22/רש"י על שמות ד׳:כ״ב

1ואמרת אל פרעה AND THOU SHALT SAY UNTO PHARAOH — When you perceive that his heart is stubborn and that he refuses to let them go speak to him thus:

2בני בכרי [ISRAEL IS] MY SON, MY FIRSTBORN — The term בכור “firstborn” denotes high dignity as, (Psalms 89:28) “I also will appoint him a בכור” (which is explained by the following words, “the highest of the kings of the earth”). This is the literal meaning; a Midrashic comment is: Here (in these words) the Holy One, blessed be He, set His seal to the sale of the birthright which Jacob had purchased from Esau (Genesis Rabbah 63:14).