מלכים ב כ:י פירוש: רש״י

And Chizkiyohu said, ”It is easier for the shade to advance ten steps.” It is easy [for the shade] to remain in its place, and stay for ten steps, it is not something that is so conspicuous. Not so, rather let the shade go back, and the event will be a wonder, for they saw that the day was turning toward evening, and it returns to become morning [again].