DEUT292 Do not excessively test a true prophet. Included in this mitzvah is that we are forbidden to test Hashem through performance of mitzvos. That is, we may not perform mitzvos with the intention of checking whether He rewards us for them. One is forbidden to make one's performance of mitzvos conditional upon receiving reward for them. Rather, mitzvos must be performed out of love for Hashem, and also out of awe and fear of Him. As to why we are forbidden to test Hashem through the performance of mitzvos--reward for mitzvos is not given in this world.