In the first episode of the third season, "The Rickshank Rickdemption", Rick tells Cornvelious Daniel about the sauce while in a brain-link simulation, and orders some from a McDonald's, as he can only eat it in his memory. Later, after watching a memory of Rick's wife and daughter apparently dying, Cornvelious comments that the sauce is "f***ing awesome."
After escaping, destroying the Galactic Federation and the Citadel of Ricks, and driving Jerry out of the family, Rick tells Morty that he wasn't motivated by saving his family, or indeed anything but "that McNugget sauce. I want that Mulan McNugget sauce, Morty. That's my series arc, Morty! If it takes nine seasons!"
In the real world, McDonald's took notice of this after being informed by fans. In response, they sent co-creator Justin Roiland a jug of the sauce, along with a note including a very in-universe explanation for how they came across it. (As opposed to simply making a small, fresh batch.) McDonald's has since also sent some to various fans, as well.
On October 1st, 2017, McDonald's confirmed in a Twitter post that the Szechuan Sauce will be returning for one day, on October 7th, 2017.
Fans came in by the hundreds, hoarding around McDonald's restaurants, only wanting Szechuan Sauce. Only a few select locations were serving Szechuan sauce, though, and as for the restaurants that didn't, fans would become outraged, throw riots, cause chaos, and make themselves look like idiots. This happened in so many places across American McDonald's restaurants, it got news coverage.
The craze over Szechuan Sauce has even become an internet meme.
On February 22nd, 2018, McDonald's announced on Twiitter that it would bring Szechuan Sauce for a wider period of time and a larger quantity, as 20 million cups of the sauce were made available starting on February 26th, 2018. Accompanying this release was a McDonald's-sponsored podcast called The Sauce which covered the Szechuan Sauce riots: several news organizations noted the podcast's hosts and guests consciously avoid referencing Rick and Morty and Dan Harmon by name, despite the fact that Rick and Morty is the entire reason the sauce suddenly became popular again.