Intro
I have written extensively on the topic of Ahmadiyya and Takfir, review some of my essays here:

https://ahmadiyyafactcheckblog.wordpress.com/2017/03/03/qasim-rashid-keeps-lying-about-his-messiahs-takfir/

https://ahmadiyyafactcheckblog.wordpress.com/2016/12/29/ibrahim-noonan-does-takfir-every-single-day/

https://ahmadiyyafactcheckblog.wordpress.com/2017/05/07/new-research-data-books-by-dr-abdul-hakeem-khan-have-been-found-vs-mga/

However, I have recently came across some partial-writings of MGA that have been translated into english, see here, https://www.alislam.org/library/books/Selected-Writings-Promised-Messiah.pdf, see page 61-62.

Obviously, MGA was writing that ALL muslims who believe in the theory of abrogation are Kafirs.  This is the earliest instance of MGA doing Takfir.