illegal participation?

[gtwbam]

What's the ruling?

Team A has the Ball on B's 5 yard line. The ball is snapped and the Quarterback goes back to pass. The wide receiver runs a down and out pattern in the end zone and inadvertently steps out of bounds (he was never forced out of bounds by the defense) The receiver realizing he stepped out of bounds returns in the end zone. The quarterback spots the same receiver in the end zone and throws him a completed pass. What's the call?