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.