This is the official rule on receptions in the NFL:
"If a player goes to the ground in the act of catching a pass (with or without contact by an opponent), he must maintain control of the ball after he touches the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete."
Basically what that means is simple. If a player goes to the ground while in the process of making a catch, he must control the ball all the way through until his momentum from the fall ends. If at any point before his momentum stops he loses control of the ball and it touches the ground, the pass is incomplete.