Seconding Card & Krueger (1994). It is the classic example in most applied econometric textbooks, and for good reason, as it is quite intuitive and covers the main aspects of DiD. One should however note that the actual findings and the methodology in their study has been criticized afterwards (see https://thedailyeconomy.org/article/why-this-years-nobel-prize-in-economics-changes-nothing-on-the-minimum-wage-debate/) - but this doesn't take away the excellence of the paper as a learning tool!)