Jamieson, Fausset and Brown on Matthew 23:24

24. Ye blind guides, which strain at a gnat--The proper rendering--as in the older English translations, and perhaps our own as it came from the translators' hands--evidently is, "strain out." It was the custom, says Trench, of the stricter Jews to strain their wine, vinegar, and other potables through linen or gauze, lest unawares they should drink down some little unclean insect therein and thus transgress (Le 11:20, 23, 41, 42)--just as the Buddhists do now in Ceylon and Hindustan--and to this custom of theirs our Lord here refers.

and swallow a camel--the largest animal the Jews knew, as the "gnat" was the smallest; both were by the law unclean.