how can I bind event on the element in iframe.html

how can I bind event on the element in iframe.html

It is does work:

parent.html
$(document).bind('sap',function(){});

iframe.html
parent.$('iframe').trigger("sap");

This is does not work:

parent.html
$(__iframe[0].contentWindow.document).bind('sap',function(){});

iframe.html
$('button1').trigger("sap");

how can I bind event on the element in iframe.html,Tks