Well, according to his wikipedia page, the exact quote is
“Above all, the prophets remind us of the moral state of a people: Few are guilty, but all are responsible.
And it cites Heschel, A.J., The Prophets (New York: Harper Collins, 1962), p. 19.
And I really like this quote too -
“When I was young, I admired clever people. Now that I am old, I admire kind people.”