Not sure if this is the fable you mean:
There was a deep, dark forest and through it walked a dog. It was quiet and cool. Suddenly, the dog came across a lion. The lion roared: It wanted to eat the dog. All at once, it began to rain and puddles formed everywhere including around the dog's feet. The dog looked down into one of the puddles and saw his own reflection and this gave him a clever idea. As the lion advanced towards him, the dog spoke quickly. He told the lion that the puddles were prisons for lions. The lion stopped moving and looked curious. The dog said that he could prove this. He told the lion to look into the puddle and that he would see in each puddle, a lion that had been imprisoned by a dog. The lion was confident as he gazed into the first puddle, and then the next. Eventually he was forced to admit that there was indeed a lion in each puddle and decided it would be best if he left the dog alone. Despite his empty stomach, the lion walked away.
There was a deep, dark forest and through it walked a dog. It was quiet and cool. Suddenly, the dog came across a lion. The lion roared: It wanted to eat the dog. All at once, it began to rain and puddles formed everywhere including around the dog's feet. The dog looked down into one of the puddles and saw his own reflection and this gave him a clever idea. As the lion advanced towards him, the dog spoke quickly. He told the lion that the puddles were prisons for lions. The lion stopped moving and looked curious. The dog said that he could prove this. He told the lion to look into the puddle and that he would see in each puddle, a lion that had been imprisoned by a dog. The lion was confident as he gazed into the first puddle, and then the next. Eventually he was forced to admit that there was indeed a lion in each puddle and decided it would be best if he left the dog alone. Despite his empty stomach, the lion walked away.