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.
I've heard Ajahn Brahm tell this story during a lecture which was recorded on YouTube, so I can confirm it's definitely told by him; however I don't remember which one it was.
The story is not (strictly) in the suttas. It is probably (very loosely) based on AN 6.42, and deals more with the disturbances of nearby lay life as opposed to the wilderness.

Nagita, there is the case where I see a monk sitting in concentration in a village dwelling. The thought occurs to me, 'Soon a monastery attendant will disturb this venerable one in some way, or a novice will, and rouse him from his concentration.' And so I am not pleased with that monk's village-dwelling.
But then there is the case where I see a monk sitting, nodding, in the wilderness. The thought occurs to me, 'Soon this venerable one will dispel his drowsiness &amp; fatigue and attend to the wilderness-perception, [his mind] unified.' And so I am pleased with that monk's wilderness-dwelling.

So in this sutta it looks more like he's worried about the auspiciousness of the environment rather than the current condition in which a monk  dwells, and his being pleased is based on his assumption that the monk will soon generate right effort, and not that he'll be heedless.
However, I think it's important to stress that Ajahn Brahm's purpose for telling this story, regardless of where he had heard it, were probably that he has many lay followers which come to the practice with high expectations for quick results. He has to teach people how to give themselves some breathing space first - and that, I believe, he does very well.