Shulchan Arukh, Orach Chayim
4. On the fifth Sabbath, which is the twenty-ninth of Adar, we take out two [Torah] scrolls — in one [the Reader] reads the weekly portion, and in the second he reads "This month shall be for you..." [Exodus 12:1–20] and the haftarah is "In the first [month], on the first of the month..." [Ezekiel 45:18–46:15]