Rashi on Exodus 12:34/רש"י על שמות י״ב:ל״ד

1טרם יחמץ BEFORE IT WAS LEAVENED — The Egyptians would not allow them to stay long enough for the dough to become leavened.

2משארתם (from the root שאר “to be left over”) denotes the leavings (what was left) of the unleavened bread and bitter herbs (Mekhilta).

3על שכמם UPON THEIR SHOULDER — Although they took many cattle with them yet they bore this upon their shoulders because the commands of God were so dear unto them (Mekhilta).