Genesis 46:3
And he said, I am God, the God of thy father: fear not to go down into Egypt; for I will there make of thee a great nation: And he said, I am God, the God of thy father: fear not to go down into Egypt; for I will there make of thee a great nation:
The time was when famine had spread from Egypt to Canaan and pressed severely on the district where Jacob lived, so that on hearing of abundant provision made by the king of Egypt ten of Jacob's sons journeyed there to buy grain. The word was spoken to Jacob at Beersheba in visions of the night while he was journeying toward Egypt with his sons, their families, and attendants. At Beersheba Jacob paused to offer grateful sacrifices and prayed for blessing and for proof that God approved the move to Egypt and would go with them.
In the word God identifies Himself as the God of Jacob's father and fathers, tells Jacob not to fear going down to settle in Egypt, and promises to make a great nation of him there. This gave Jacob the evidence he sought that God would go with them. With this assurance God also promised that He Himself would go down with Jacob into Egypt, would surely bring him up again, and that Joseph would be with him at his death.
The move was necessary because the promise of descendants as numerous as the stars had not yet been fulfilled and Canaan could not then support the growth of such a nation. Its powerful inhabitants were not yet to be displaced, and living among them risked drawing Israel into idolatry. Egypt was chosen because its fertile, well-watered land allowed rapid growth, while Egyptian antipathy toward shepherds would help Israel remain a separate people and avoid Egyptian idolatry.