The following story appears in the Talmud ( I think ), but I don't know where:
There was a Rabbi who saw a man and his son come along, and the father sent his son up a tree to do shiluach haKen. The son falls from the tree and dies, and the Rabbi becomes an apikores, iirc citing that the son died after performing two mitzvot whose reward is lengthening of days. The son of this Rabbi, who is also a Rabbi comes and says his father didn't know the principle that there is only reward in olam haba.
Where does this story appear?