This is a well-known quote from President Reagan, but it isn't always quoted at length, and the source is not always given. I remember even finding people online who questioned whether President Reagan ever said it. (I found it on page 38 of "A Time For Choosing," a book published by Regnery Gateway in the early 1980s.)
"In this land occurred the only true revolution in man's history. All other revolutions simply exchanged one set of rulers for another. Here for the first time the Founding Fathers--that little band of men so advanced beyond their time that the world has never seen their like since--evolved a government based on the idea that you and I have God-given right and ability within ourselves to determine our own destiny. Freedom is never more than one generation away from extinction--we didn't pass it on to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children's children what it was once like in the United States when men were free."
Reagan said something similar when he was inaugurated as Governor of California:
"Perhaps you and I have lived with this miracle too long to be properly appreciative. Freedom is a fragile thing and is never more than one generation away from extinction. It is not ours by inheritance; it must be fought for and defended constantly by each generation, for it comes only once to a people. Those who have known freedom and then lost it have never known it again. Knowing this, it is hard to explain those who even today would question the people's capacity for self-rule. Will they answer this: if no one among us is capable of governing himself, then who among us has the capacity to govern someone else?"