Where did Descartes write, "With me everything turns into mathematics."

I have been trying to source a famous quote of Descartes, "Omnia apud me mathematica fiunt." or "With me everything turns into mathematics."

I cannot find a source for this. The English e-books I have, nor the the Latin version on Google books.

It's scattered all over the web and "turns into mathematics" shows up in Google's N-Gram viewer starting in 1930. None ever cite the book it came from, let alone the chapter.

This quote is said to be from a letter of Descartes to Mersenne dated 11th March 1640 (Correspondence 1640 - 1643, page 36, note to line 7). The original sentence was "[...] apud me omnia fiunt Mathematicè in Natura [...]".

A version translated from here along with some discussion regarding the translation can (thanks to this Q/A) be found on Wikiquote.

An image of the original page is online, but I could not include more than a snippet here: