offside question

[Kmet]

I am a basketball offical (and soccer dad) trying to make sure I understand the nuances of offside. My question - Player A1 has the ball in a legal position moving down the right side of the field. Only Team B's goalkeeper remains between him and the goal. The keeper closes down the angle so A1 makes a backwards pass to A2 on the left side who is behind A1 but ahead of Team B defenders. Is this an offside pass or legal since the pass is backwards? If it is legal what is the citation in the federation rule book. Thank you.