424. Are You People Never Satisfied? The prohibition against testing a prophet excessively
You shall not test Hashem your God… (Deuteronomy 6:16)
If someone claims to be a prophet, it may be reasonable to ask for some corroboration of this claim, but we are not to make him jump through hoops for us. After all, the prophet is telling us to return to God and to follow His Torah, which are things we have to do anyway! (This is the conceptual opposite of the case of the false prophet, in Mitzvah 517. There, no matter how many "miracles" he may work, we are not to listen to a "prophet" who tells us to go against the Torah.)
Pushing a prophet doesn't end well for anyone. First of all, prophets have enemies. These could be prophet-wannabes or just people for whom the prophet's message is "bad for business." These people would vex the prophets non-stop if we were allowed to ask for constant proof. Also, prophecy isn't a tap that can be turned off and on at will. It's not as if they could work wonders at will. They relayed whatever messages and performed whatever acts that God told them to, but they couldn't necessarily perform on demand - nor would it be respectful for us to expect them to! (A prophet is not a magician hired for a 10-year-old's birthday party!) And so, once a prophet has established his credentials with two or three accurate predictions, we may not demand a miracle as proof. (If a prophet foretells destruction and it does not occur, he is not disqualified as a prophet since prophecies of doom may be averted through prayer and repentance. If he foretold good things and they do not come true, the "prophet" is disqualified since God always fulfills His good messages.)
This mitzvah also includes not testing God himself. For example, one may not say, "Okay, God, here's $500 for charity, now heal so-and-so." One may, however, pray, learn or donate charity in someone's merits with no strings attached. In such a case, the money would be considered a gift to charity regardless of whether or not so-and-so recovers, and therefore not a test.
There's one exception to the rule about not testing God: if we scrupulously observes the laws of tithing, we are promised that God will enrich us. The Book of Malachi explicitly tells us that we should "test" God in this (3:10). Otherwise, it would be pretty pointless for us to even try testing God, as our reward for keeping mitzvos in the Next World, so it's pretty unobservable from here.
This mitzvah applies to men and women in all times and places. It is discussed in the Talmud in tractate Sanhedrin (89a-b). It is codified in the Mishneh Torah in the tenth chapter of Hilchos Yesodei HaTorah. This mitzvah is #64 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos. It is not listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.