During the golden Abbasid period, one of the scholars in Baghdad, the capital of Muslim caliphate at that time, was reputed to hold knowledge in high esteem.
One day an acquaintance met the great scholar and said, "Do you know what I just heard about your friend?"
"Hold on a minute," the scholar replied. "Before telling me anything I'd like you to pass a little test. It's called the Triple Filter Test."
"That's right," the scholar continued. "Before you talk to me about my friend it might be a good idea to take a moment and filter what you're going to say.
That's why I call it the triple filter test.
The first filter is Truth. Have you made absolutely sure that what you are about to tell me is true?"
"No," the man said, "actually I just heard about it and.."
"All right," said the scholar. "So you don't really know if it's true or not.
Now let's try the second filter, the filter of goodness. Is what you are about to tell me about my friend something good?"
"No, on the contrary..."
"So," the scholar continued, "you want to tell me something bad about him, but you're not certain it's true. You may still pass the test though, because there's one filter left: the filter of usefulness.
Is what you want to tell me about my friend going to be useful to me?"
"No, not really."
"Well," concluded the scholar, "if what you want to tell me is neither true nor good nor even useful, why tell it to me at all?"
"O ye who believe! Let not some men among you laugh at others: It may be that the (latter) are better than the (former)" ... (to end of surah) Hujurat 49.12
"And spy not on each other behind their backs..." (to end of surah) Hujurat 49.13