Money Rock

Every day, one random rock in your town will spurt out up to 8,500 bells as sacks of 100, 200, 400, 800, 1000, 2000 and 4000 bells, in that order. To get them you must hit the rock with a shovel or an axe, although you will bounce back and not be able to collect all of the sacks, and since the more valuable ones come last, you'll get less money. To prevent this, you must dig two holes. Stand diagonally next to the rock and dig one above/below you, whichever is further away from the rock, then one to your right or left, again, whichever is further from the rock. Now when you hit it, the holes will stop you bouncing back.

From Animal Crossing Wild World Wiki, a Wikia wiki.