Is it a good sign if someone sneezes during shamona esrei? Is it just during shemoneh esrei or during any prayer? What about having gas? Can you explain the reasons?
The Gemara (Berachos 24b) writes that sneezing is a good sign during prayer, meaning during the Shemoneh Esrei prayer. Passing gas is a bad sign, and it is forbidden to pass gas during the Amidah prayer.
The source for these statements is the Gemara, as ruled by the Tur and the Shulchan Aruch (103). The reason given for why sneezing is a good sign is that just as the person has been “relieved” below, so he is “relieved above.”
This does not apply to flatulence, possibly because flatulence indicates a need of relieving oneself, and therefore indicates that the person is not in the right bodily condition for prayer, and also because it is considered a base physical function and inappropriate for somebody standing before the king.
Another point to note is that flatulence can be controlled, whereas sneezing cannot.