"…And G‑d said unto Abram: Know of a surety that Your seed shall be a stranger in a land not is not theirs; and shall serve them; and they shall afflict them four hundred years. And also that nation, whom they shall serve, Slavery in Egypt will I judge; and afterwards shall they come out with great substance."

(Genesis 15: 13,14)

"….the 400 years of exile and affliction began with the birth of Isaac. When Isaac was sixty years of age his son Jacob was born, and at the age of 130 Jacob came to Egypt, that makes 190 years. In Egypt the children of Israel actually spent 210 years, that makes 400 years in all".

(Rashi)

"Even on that selfsame day it came to pass that all the hosts of the L-rd went out from Egypt."

(Exodus 12:41)

"When the end came G‑d did not delay them even to the extent of a wink: On the 15th day of Nissan Isaac was born and on the 15th day of Nissan (400 years later) the children of Israel were liberated from Egypt."

(Midrash Tanchuma)