Most likely read on FF.net. I can't remember if it was a de-ageing accident or if it was a "Snape rescues/adopts Harry as a baby".
The most prominent portion of the story that I can only remember standing out was the detail of Snape's living quarters which had now been transformed to accommodate the baby Potter. Probably from Dumbledore's meddling, there was now a giant, lumpy, disgustingly soft blanket (play-mat) in the middle/center of his room/living room/floor. Just one look at it was repulsive to him (Snape), and it looked so soft it looked as if it would make him sink into the floor if he even dared to step foot on it.