Rashi on Deuteronomy 16:6/רש"י על דברים ט״ז:ו׳

1בערב כבוא השמש מועד צאתך ממצרים [THERE THOU SHALT SACRIFICE THE PASSOVER] AT EVEN, AT THE GOING DOWN OF THE SUN, AT THE APPOINTED TIME THAT THOU CAME FORTH OUT OF EGYPT — But these are three different points of time! The explanation is: at evening, i.e. from the sixth hour (reckoning from six o’clock in the morning) and onwards you shall slaughter it (תזבח בערב); and when the sun goes down you shall eat it, and at the time when you left Egypt (in the early morning) you shall burn it — that is to say, if in the morning there is any flesh left, it becomes what is technically termed נותר and must be removed to the “place of burning” (not actually burnt, for this is not permissible on a festival) (Sifrei Devarim 133:1; cf. Berakhot 9a).