The 33rd prohibition is that we are forbidden from acting on the basis of omens [l'nachesh],1 for example when people say, "Since I just come back from a trip, my will won't be carried out"2; or, "The first thing I saw today was such-and-such — certainly I will earn something today." This kind of behavior is very widespread among the masses of the foolish nations.3

Anyone who acts based on an omen is punished by lashes, in accordance with G‑d's statement (exalted be He),4 "Among you there shall not be found anyone...who divines auspicious times, who divines by omens [menachesh]." This prohibition is repeated in G‑d's statement,5 "Do not act on the basis of omens."

In the words of the Sifri, "Examples of a menachesh are one who says, 'my bread fell from my mouth'6; 'my staff fell from my hand'; 'a snake passed on my right'; 'a fox passed on my left.7'" The Sifra says, "Examples of the prohibition, 'Do not act on the basis of omens,' are those who divine omens from a weasel, birds, stars, etc."

The details of this mitzvah have been explained in the seventh chapter of Shabbos,8 and in Tosefta Shabbos.9