It’s in a novel—The Shattering, I think. But it often gets overstated. It was more like Thrall wanted the orcs to work hard to build a life in Durotar as a positive outlet for their energy. I don’t think Thrall himself ever uses the word “penance” (though he does use “atonement”). The word penance comes from some other character.
The very first story of why Thrall chose Durotar as the orcs’ new home was that it reminded them of their homeland. This got retconned when TBC came out and most of Draenor didn’t resemble Durotar very closely.
Here’s a link to a post with quotes from The Shattering:
It’s also worth noting that all this talk of “penance” comes up right at the time the devs are trying to give Garrosh a motivation to be warlike. Probably not a coincidence.