You should ask MUCH sooner. You should not speed up on the approach to the jump, that teaches them to rush. Instead, start your canter maybe half way around the ring and get the canter the speed you want, then breathe deeply and relax on the approach
What jillyann is talking about is a ground pole that's put before a jump to give a green horse an idea of where to take off. In your case, I would put one or two poles further out so your horse has to stride into them and concentrate on them, not the jump.
Horses rush for several reasons, the main one being that they are not confident jumping! Make sure your jump is small, establish a good canter MUCH farther out so your horse can get a nice rhythm going, and DON'T PULL. This just makes things worse. Fix the reason why your horse is rushing, instead of just hauling on their sensitive mouths. Make sure you don't jump ahead or behind, and make sure you're not catching him in the mouth or bumping down on his back. All of these things can make the horse VERY uncomfortable with jumping, and then he'll just want to rush through all of the jumps because you hurt him when he tries.
What everyone is trying to say, and to put it into sense, is that you are asking WAY to late! 2 strides out, and the horse is still trying to balance at their canter to get a nice rythm going so they can find their distance and get over the jump safely. You are asking for the canter when the horse has already seen his distance and is ready to jump, you change his balance, which changes where he is to take off. Canter around the ring once or twice and get a nice stedy rythm going, and then go to the jump. If he starts to quicken his pace, half halt, and have him re balance. A ground pole is a very good idea, because it will help the horse see his distance better.