Rambam - Sefer HaMitzvos: Negative Mitzvah 157: Negative Mitzvah 157

Negative Mitzvah 157: It is forbidden to go back on our word
Numbers 30:3 "He shall not break his word, he shall do according to all that proceeds out of his mouth"

Sometimes, we are not careful enough about what we say and then, we are forced to explain ourselves because of a misunderstanding.

Often we'll hear someone remark:

"Oh! I wasn't even thinking when I said that!"

"Did I say that?" I didn't really mean it."

"I just said the first thing that came to my mind!"

The Torah teaches us to be responsible for what we say.

If a person made a commitment to do something or take upon himself some job or responsibility, he should not go back on his word.

When Elijah informed King Chizkia of the heavenly decree that he would die, the king replied, "I have a tradition from my ancestor, David: Even when the sword is drawn over your neck do not refrain from pleading for mercy from Above."
There is hope, and there is trust in G-d -- and they are two distinct attitudes. Hope is when there is something to latch on to, some glimmer of a chance. The drowning man, they say, will grab even on to straw in an attempt to save his life. Trust in G-d is even when there is nothing in which to hope. The decree is sealed. The sword is drawn over your neck. By all laws of nature there is no way out. But the One who runs the show doesn't need any props.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com