הלכה על פסחים 69:3

The commandment of eating matsah: To eat matsah-bread that is made from a species of grain on the night of the fifteenth of Nissan (Pesachim 35a), as it is stated (Exodus 12:18), "in the evening, you shall eat matsot." And the understanding [of "evening"] is the night of the fifteenth of Nissan - whether it be at a time when the Pesach sacrifice is present or whether it be at a time that it is not present.