It's from the Order for the Burial of the Dead in the Book of Common Prayer, and is said at the graveside.

Man that is born of a woman hath but a short time to live, and is full of misery. He cometh up, and is cut down, like a flower; he fleeth as it were a shadow, and never continueth in one stay.
In the midst of life we are in death: of whom may we seek for succour, but of thee, O Lord, who for our sins art justly displeased?

The well-known "earth to earth" bit doesn't follow directly on from this, as CMJ_Trans implies in his quotation - that comes in the next prayer when the coffin is lowered into the ground.