NUM250 R. Simeon b. El'azar said: … do not question [your fellow] at the time he makes a vow. Pirkei Avot, Perek IV, mishnah 23. Whence do you learn this?--From Moses. When he said to the Israelites, "Listen now, you rebels," [this verse] the Holy One vowed that he would not enter the [promised] land.… Said Moses, "This is a time of oath – taking; I have no need to speak now." Forty years he waited, and then he began to plead before Him. Said the Holy One to him, "Because you waited, go up to the top of Pisgah [and behold it with your eyes]" (Deuteronomy 3:27). This much his patience availed.