The line is from the following paragraph in Chapter 1 of SK’s Sickness Unto Death:

“The self is a relation which relates itself to its own self, or it is that in the relation by which the relation relates itself to its own self; the self is not the relation but is the relation’s relating itself to its own self. The self is not the relation but is the relation’s relating itself to its own self. Man is a synthesis of the infinite and the finite, of the temporal and the eternal, of freedom and necessity, in short, a synthesis. A synthesis is a relation between two. Considered in this way, man is not yet a self."