Shulchan Arukh, Orach Chayim
2. On the second Sabbath we take out two [Torah] scrolls — in one [the Reader] reads the weekly portion, and in the second he reads "Remember what Amalek did to you..." [Deuteronomy 25:17–19], and the haftarah is "I took account of what Amalek did to you..." [I Samuel 15:1–34].