Microsoft’s original source code

Microsoft got its start by Bill Gates doing some dumpster diving. Back then software wasn't seen as valuable thing, only hardware was. Source code wasn't something to be protected, so printouts of code would be thrown in trash. And that's where Bill Gates found the source code for Basic interpreter, which he ported and it became the first Microsoft product.

https://americanhistory.si.edu/comphist/gates.htm

https://paulallen.com/Futurist/Microsoft.aspx

I couldn't find the precise reference that mentions that they found the source code for the Basic interpreter and just "copied/ported" it. I did read they'd go "dumpster diving" to learn assembly. But not that they found and just ported the source code. Where is it?