From The Gulag Archipelago Two, (Harper & Row, 1975), p. 615:
"It was granted me to carry away from my prison years on my bent back, which nearly broke beneath its load, this essential experience: how a human being becomes evil and how good. In the intoxication of youthful successes I had felt myself to be infallible, and I was therefore cruel. In the surfeit of power I was a murderer, and an oppressor. In my most evil moments I was convinced that I was doing good and I was well supplied with systematic arguments. And it was only when I lay there on rotting prison straw that I sensed within myself the first stirrings of good. Gradually it was disclosed to me that the line separating good and evil passes not through states, nor between classes, nor between political parties either - but right through every human heart - and through all human hearts. This line shifts. Inside us, it oscillates with the years. And even within hearts overwhelmed by evil, one small bridgehead of good is retained. And even in the best of all hearts, there remains . . . an unuprooted small corner of evil."
"The Soul and Barbed Wire", Chapter One, "The Ascent."
Thanks b.g. for helping me dig this, my favorite Solzhenitsyn quote, back up.