Surah Isra': 36 (do not speak of which you have no knowledge)

In the name of Allah, The Most Gracious, The Most Merciful.

وَلاَ تَقْفُ مَا لَيْسَ لَكَ بِهِ عِلْمٌ إِنَّ السَّمْعَ وَالْبَصَرَ وَالْفُؤَادَ كُلُّ أُولـئِكَ كَانَ عَنْهُ مَسْؤُولاً
"And follow not that of which you have no knowledge. Verily, the hearing, and the sight, and the heart of each of those ones will be questioned (by Allah)." (Surah Isra': 36)

Lessons:
-Ibn Abbas said: This means do not say anything of which you have no knowledge.
-Al-`Awfi said: "Do not accuse anyone of that of which you have no knowledge.''
-Muhammad bin Al-Hanafiyyah said: "It means bearing false witness.''
-Qatadah said: "Do not say, `I have seen', when you did not see anything, or `I have heard', when you did not hear anything, or `I know', when you do not know, for Allah will ask you about all of that.''
- In short, it means do not speak without knowledge, and on the basis of mere suspicions.
-A hadeeth from Sunan Abi Dawud: Beware of suspicion, for suspicion is the falsest of speech.
-Allah says: Avoid much suspicion; indeed some suspicions are sins. (Surah Al-Hujaraat:12)

And Allah knows best.