מדרש על שמות 12:15

5) R. Shimon b. Elazar says: In one place (Devarim 16:8) it is written "Six days shall you eat matzoth," and, in another, (Shemoth 12:15) "Seven days shall you eat matzoth"! How is this to be resolved? Matzoh which cannot eat seven days from the new grain, (permission to eat of the new grain [chadash] beginning with the bringing of the omer on the second day of the festival), you can eat, seven from the old grain and six from the new. How, then, must I understand "from the morrow of the Sabbath? As from the morrow of the festival.