I don't know the original source, but it fairly common financial advice. Here is one source [1] but a simple DuckDuckGo search or Google Search will show many others. Some are starting to recommended 3-4x due to low interest which as I said in other comments I do not agree with.
Source? I have heard many boomers talk about how the advice was to pay no more than 4x income. Obviously highly unlikely in this day and age.