I have heard it said that only the mitzvot of kibbud av v'eim (honoring parents) and shiluach haken (sending away the mother bird before taking eggs) are given with an explicit reward (that your days may be lengthened). Often times this is cited in relation to the story of Acher in the Gemara, although it doesn't seem to be there.
In fact, I can't find a source for it anywhere. And it would seem that Devarim 25:15 would be a third case (specifying honest weights and measures), so it would also seem to be wrong. Yet it also seems to be something that "everyone" knows.
Can anyone help track down a written source for the popular belief?