Berachot 2 - 8
"Teach your tongue to say 'I don't know' lest you be exposed as a liar."
The source for this advice is the manner in which Moshe warned Pharaoh that the plague of the Egyptian firstborn would take place at midnight. He used the term "around midnight" although he knew that G-d would perform this miracle exactly at midnight. The reason for his caution was a fear that Pharaoh's astrologers would be mistaken in their calculations of midnight and claim that Moshe was a liar.
- Berachot 4a