When you push a creature, each square you move it must place it farther away from you.

•     Two-Dimensional: Forced movement is normally two-dimensional; all the squares of the movement must be on the same horizontal plane. Forced movement can become three-dimensional when the tar-get is flying, is moved through a substance such as water, or is on a non-horizontal surface, such as an incline, that supports it. This means an earthbound target cannot normally be pushed to a square in the air, but a hovering target can be. Similarly, a target can be pulled down a flight of stairs, and it can be slid in any direction underwater.

Published in Player's Handbook, page(s) 285, Monster Manual, page(s) 282.