inbound

[xxssmen]

I was playing in a rec. league last night and after a made shot I proceed to inbound the ball by stepping 1 foot out and the other foot lifted in the air so I was straddling the line with 1 foot out and the other in the air.
One of the official tells me to be legally out of bound which I thought I was when I step out and nothing else was touching the playing court, so he gave me an warning for it when I did it again, so when I ask him the ruling on that, he said that I had to be completely across the boundary line.
Can anyone tell me where in the rule book that say that?