Many people have small siddurs that they use for Mincha and Marriv which they keep in their wallets. I happen to keep my wallet in my back pocket. If I pick up a siddur like this, am I able to keep it in my wallet (which is in my back pocket)?
According to Rav David Law, which now is the Rav Rashi of Israel, no.
His source is in Berachot (18a) which says that someone who brings bones or a Sefer Torah from place to place on his animal should not sit on them even if they are in a container (דקסיא) because it is disrespectful to the bones,the Sefer Torah and, by implication, the siddur.