Kiddushin 39 - Reward for a Mitzvah is Another Mitzvah
Orlah (prohibition of the first 3 years of tree's fruit) reads as follows: in Israel it is forbidden, if in doubt, it is also forbidden. Outside of Israel one can go to the field where the Orlah tree grows and buy the fruit, provided that one does not see it being picked.
One who performs an extra mitzvah is getting the goodness of performing another mitzvah, and his life is extended to do it. Thus, the reward for a mitzvah is only another mitzvah, except for respect for the parents and sending away the mother bird, for which long life is promised..