I asked the following question:
“The “siman hayadua” for the seder of brachos for achila is brought down as מג”ע א”ש What’s the earliest makor for this siman?”
I received the following answer:
“Answer: No idea. Better ask the historians….”
Cute. I asked and they didn’t know. So let’s try again. Where is at least one makor in poskim (achronim or whenever) that isn’t a modern day likut Sefer. If there even is a makor…
I don’t know if there is an early source for this.
The siman is based on the rulings of the Magen Avraham (211:13).
Note that the siman is true for minhag Ashkenaz, but not necessarily for Sefard. See Shetilei Zeitim, Vol. 2, 211, Zayit Ra’anan 21.
I guess you asked the wrong historians…