The 199th prohibition is that we are forbidden to eat chometz [leaven] on the 14th [of Nissan] after noon.

The source of this commandment is G‑d's statement,1 "Do not eat any chometz with it."

This expression ["with it"] refers to the Pesach lamb which we are required to sacrifice between noon and sundown on the 14th [of Nissan]. Therefore, the verse means that when the time to bring the sacrifice begins [i.e. noon], you may not eat chometz.

Tractate Pesachim2 says, "What is the source of the prohibition not to eat chometz after the sixth hour [i.e. noon]? The verse, 'Do not eat any chometz with it.' " It also says there,3 "According to all opinions, [eating chometz] after the sixth hour is a Biblical prohibition." This is the version of all the reliable copies which were read by the Talmudic Sages.4

The reason for the prohibition of eating chometz during the sixth hour5 is said there:6 "The Sages made an additional prohibition in order that people not come to a Biblical prohibition." Anyone who transgresses and eats chometz after noon is punished by lashes.

The details of this commandment are explained in the beginning of tractate Pesachim.