<PERSON>'s text in I. Asimov, which was the basis for the Wikipedia summary quoted by <PERSON>, doesn't exactly state when he made the decision to merge the two universes, but it says that he began to implement the merge with Robots and Empire (1985):
[...] my pleasure with The Robots of Dawn led me to write a fourth robot novel. In the fourth book, <PERSON> would be dead, but I had already decided that the robot, <PERSON>, was the real hero of the series, and he would continue to function.
Still, the fact that my robots were becoming increasingly advanced with each robot book, made it seem stranger and stranger that there were no robots in my Foundation series.
Carefully, I worked out a reason for it and, in doing so, I could see that it was going to be necessary to tie my robot novels and my Foundation novels together into a single series. I intended to begin that process with the upcoming fourth robot novel, and to give a hint of my intention I was going to call it Robots and Empire.
[...] To my surprise and considerable chagrin, the del Reys argued strongly against my plan to fuse the two series into one. They said the readers would prefer to have the two separate [...]
I stumbled away very dispirited and explained the situation to <PERSON>. [...] She said, “What is it you want to do, <PERSON>?”
I said miserably, “I want to tie the series together.”
“You’re the writer. 26
Do it.” [...] I went ahead and wrote Robots and Empire and clearly began the process of fusing the two series.
I. <PERSON>, 1994, chapter 145
However, this is inconsistent with what <PERSON> wrote in an earlier essay! Here he claimed that he started the process with The Robots of Dawn (1983):
In Foundation’s Edge, I made a point of referring to various non-Foundation novels of mine and indicating how they might fit into the Foundation universe. (A few of the critics disapproved of this, but I’m darned if I know why. They are my books and I can do as I wish with them, I should think.)
A major embarrassment were the <PERSON> novels which can in no way be fitted into the Foundation Universe. The <PERSON> novels contain societies based on strains of humanity with life-spans of up to four centuries and supported by elaborate systems of robots. The Foundation novels have ordinary short-lived strains of humanity only, and no robots at all.
In Foundation’s Edge, I finally mentioned robots, and in The Robots of Dawn I amused myself by letting the plot serve the function of pointing the Lije Baley universe in the direction of the Foundation universe.
My Projects, IASFM October 1983
I think the apparent inconsistency can be explained by seeing this as a gradual process: In Foundation’s Edge (1982), <PERSON> mentioned robots, but those were not necessarily the same ones as in the robot novels. In The Robots of Dawn (1983), he began to converge the two series, and in Robots and Empire (1985), he made the merge explicit. 26