Genesis: 18:4 – Let a little water be brought and wash your feet and recline under the tree.
Gemora: Bava Metziah 86(b): They [the travellers] protested to him [Abraham], ‘Dost thou suspect us of being Arabs, who worship the dust on their feet? Ishmael has already issued from thee.’
Torah Temimah Colloquial Translation on Note #17:
In other words, this was Abraham’s punishment that Ishmael descended from him. One needs to say that it wasn’t actually that Abraham suspected them of being Arabs. Behold, Arabs didn’t even exist yet before Ishmael. Rather it means to say that Abraham suspected them of doing actions like those that Arabs would do in the future.
The reason why the Gemora comments on this posuk is because Abraham prefaced “washing the feet” to “recline under the tree” which is the opposite of what is normally said. We know this to be the case by seeing what Lot did and Laban did. They said first “rest” and then “wash”. However, Abraham did the opposite because he was very careful (מקפיד) about dust because it was used as idol worship by some. The Maharsha notes that the response of the travelers noted above is not explicitly mentioned in the bible. We would not actually not know about the traveler’s response except for the fact that Abraham was punished by having Ishmael descend from him, a nation who worshipped the dust of their feet.
We believe [Gemora Shabbos 97(a)] that one who is suspicious of innocent people will be stricken in his body.
DBS Note: The Torah Temimah quotes sections of the Talmud that are often not known to those who do not assiduously study Gemora. It seems amazing that Abraham is punished for being extra cautious and trying hard to avoid coming into contact with idol worship. Yet the point of the Gemora, and the Torah Temimah’s point in quoting this Gemora, seems to be to emphasize that it is wrong to be extra cautious with stringencies if it entails suspecting innocent people of transgressions for which they are not guilty.
By the way, an amazingly similar comment regarding Moshe’s actions is mentioned in the Torah Temimah’s comment on Shmos 4:1. Our translation of that is located here: http://temimahblog.com/?p=30