Midrash su Salmi 146:5

Now Jacob saw that there was grain in Egypt (Gen. 42:1). Scripture states elsewhere in reference to this verse: Happy is He whose help is the God of Jacob (Ps. 146:5). Why does this verse say the God of Jacob and not “the God of Abraham” or “the God of Isaac”? Because the Holy One, blessed be He, stood at the side of Jacob but not at the side of either Abraham or Isaac, as is said: And, behold, the Lord stood beside him (Gen. 28:13).

R. Simeon explained: A king never stands in his field while it is being sowed or plowed or hoed, he does so only while the grain is being stacked. Abraham hoed, as it is said: Arise, walk through the land (ibid. 13:17), and Isaac sowed, as it is said: And Isaac sowed in the land (ibid. 26:12). The King did not stand beside anyone until Jacob came, for he stacked the first fruits, as it is said: Israel is the Lord’s hallowed portion, and the first fruits of the increase (Jer. 2:3). Then the Holy One, blessed be He, stood beside him (Gen. 28:13). Therefore, Happy is he whose help is the God of Jacob, whose hope is in the Lord his God (Ps. 146:5). Resh Lakish declared: Whose hope is in the Lord his God refers to Joseph, who was the hope of the world while dwelling in Egypt. The Holy One, blessed be He, revealed to Jacob that his hope was in Egypt, as it is said: Now Jacob saw that there was hope in Egypt (reading sever, “hope,” for shever, “grain”).

Another interpretation (of Gen. 42:1): THEN JACOB SAW THAT THERE WAS GRAIN (ShBR)…. This text is related (to Ps. 146:5): BLESSED IS THE ONE WHOSE HELP IS THE GOD OF JACOB, WHOSE HOPE (SBR) IS IN THE LORD HIS GOD.²¹Gen. R. 91:1, 6; cf. Tanh., Gen. 10:5. The Holy One showed him that Joseph was alive.