LEAVEN BELONGING TO A GENTILE OVER WHICH PASSOVER HAS PASSED IS PERMITTED FOR USE; BUT THAT OF AN ISRAELITE IS FORBIDDEN FOR USE, BECAUSE IT IS SAID, NEITHER SHALL THERE BE LEAVEN SEEN WITH THEE. (Ex. 12:10)
Now that seems fairly clear – if leaven had remained with an Israelite, even though it was not used during Passover, it cannot be used after for it had broken the commandment as stated.
The Rabbis ask who the author of this Mishnah is and begin by stating it cannot be R. Judah, R. Simon or R. Jose the Galilean because each used this verse to elucidate another law.
R. Judah, for example, points out that there are 3 laws of leaven – that is 3 verses:
There shall no leavened bread be eaten; (Ex. 13:3) Ye shall eat nothing leavened; (Ex. 12:20) and Thou shalt eat no leavened bread with it [the Passover sacrifice] (Deut. 16:3).
One refers to before its time; another to after its time; and the third to during its time.
Others interpret differently. This one, I found fascinating:
R. Jose the Galilean said: How do we know that at the Passover of Egypt its [prohibition of] leaven was in force one day only? Because it is said, ‘There shall no leavened bread be eaten’, and in proximity thereto [is written], This day ye go forth.
Quite an interesting idea: according to this view (not universally accepted), the first Passover was observed one day only and with the Passover sacrifice (and the Exodus!). All subsequent ones were observed in full and without hametz throughout.