(Exodus 13:5) "which He swore to your forefathers": Where did He swear it to your forefathers? Abraham — (Genesis 15:18) "On that day the L–rd made a covenant with Abraham." Isaac — (Ibid. 26:3) "Live in this land." Jacob — (Ibid. 28:13) "the land on which you are lying, etc."... "Then you shall perform this service": As the service that you performed in Egypt so shall you perform in all the generations. These are the words of R. Nathan.
God Swore the Land to Each Patriarch Individually
Mekhilta Tractate Pischa 17:4