Where is the story that Buddha was worried about a monk who meditated with a straight back?

I'm trying to find a story in the Pali canon (or maybe not) reportedly told by Ajahn Brahm that goes like this (copied from here ): > Buddha was wandering along with Ananda when they came across a monk > sitting under a tree in meditation. The monk was sat on the ground > with a straight back, his hands were folded, and his head and neck at > just the right angle. He was deep in meditation and had been for some > time. The Buddha turned to Ananda and said “I’m worried about that > monk.” > > A few minutes later they came across another monk sitting under a tree > in meditation. He was on a comfy cushion, his back was bent forward > and he’d fallen asleep. Every now and then he’d wake up only to nod > off again. He was even snoring. The Buddha turned to Ananda and said > “this monk I’m not worried about at all, he’s doing just fine.” I've been googling around but found nothing.