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 explination 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.