Where in Undertale does it ever say that Sans has "karmic retribution"?

I've noticed quite a lot in comment regions on youtube and such mentioning that Sans has this thing called karmic retribution which means people who have committed sins take more damage from him. Now, I've never played undertale (far too difficult for me) but I have watched 2 entire playthroughs of it all and try to keep tabs on it heavily. I'm confused where this comes from in game. I've never seen Sans or Flowey ever mention this (I would only assume to come from them as nobody else would have first... petal/bone experience).

Now I know what they are referring to (his poison attack that drains health), but I am confused where they get the idea that it's karma and not just a poison attack.

It doesn't necessarily say "karmic retribution" in the game. See the wiki page:

All of Sans's attacks use KARMA (KR). This effect removes invincibility frames, and, in turn, delays damage input. HP cannot be reduced below 1 while KARMA is in effect and is depleted in the place of HP when taking damage; if the protagonist suffers otherwise fatal damage while KARMA is in effect, it effectively becomes a decaying second health bar. Purple is the mode and color of the purple SOUL and is associated with Trap Mode and perseverance, but it is unknown if this is associated with KARMA.

Karmic Retribution is a common phrase that refers to when someone gets what is coming to them. Since you only fight Sans if you've been killing NPCs, his attacks use KARMA, and he is by far the hardest fight in the entire game, it makes sense that people would refer to it as such, even if that's not technically what it is called in-game.

At least one reason it's called KARMA is because of specific flavor text that appears when you have enough KARMA damage racked up. The flavor texts are as follows:

You felt your sins crawling on your back. (0-10 KARMA damage)

You felt your sins weighing on your neck. (10-20 KARMA damage)

KARMA coursing through your veins. (20-30 KARMA damage)

Doomed to death of KARMA! (30-40 KARMA damage, according to wiki, but no longer used in-game because of programming errors)

As you can see, KARMA is actually named in-game, which is where part of it stems from. Now, as to where people get the 'Sans can only use this against people who sinned' thing from, I don't know. I personally don't think KARMA really means the same karma people speak of (that whole impersonal force which comes back to bite you in the butt when you screw up), since LOVE and EXP are acronyms who don't mean what we originally think either. The fact it's used in all caps makes me think it's some sort of acronym... maybe something to do with killing, since both words starts with a 'k'. However, it seems no one knows what it really means, or if someone does, I've never heard of it.

The only thing I can think of is the 'sins crawling up your back' thing, which yeah, makes it sound like KARMA is the same thing as karma. Maybe it's possible Sans can actually only use this on people who have sinned, but to my knowledge, that hasn't been confirmed.

Karmic retribution does mean that whoever has commited more sins will get more bad things in return and sans does use it in game. On a genocide run where you kill all the monsters in the underground and you commit the most sin is the only place where you get to fight sans and see karma in effect. Now, karmic retribution is never mentioned in game, BUT... From the flavour text provided in fight (you feel your sins crawling on your back) it definitely provides indication that in game karma and actual karma are the same or similar, so, according to all of this, karmic retribution is EXTREMELY likely to be canon, however since it is not mentioned in game, it is not confirmed. However, due to all the evidence supporting its existence, you could consider it canon.