This is the video link（the story starts at: 20:56）:
In this interview Bhikkhu Analayo says:
“And it was years later that I found there is a story about this in the commentaries. The story is and now we make a little bit of fun of monks if I’m allowed to do that. So there are monks and nuns sitting listening to a Dhamma discourse. The monk should be aware of what he does when he stretches or bends his arm but he is not. So he stretches his arm and he touches a nun and he touches her on the breast. He pulls her back and she pulls it back. You can imagine the rest of the story and you can imagine why a monk who recites that part will feel giggling.”