Do not speak without Knowledge

Allah says:

وَلَا تَقْفُ مَا لَيْسَ لَكَ بِهِ عِلْمٌ ۚ إِنَّ السَّمْعَ وَالْبَصَرَ وَالْفُؤَادَ كُلُّ أُولَٰئِكَ كَانَ عَنْهُ مَسْئُولًا

"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)."  [Quran 17:36]

`Ali bin Abi Talhah reported that 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 conclusion, what they said means that Allah forbids speaking without knowledge and [forbids speaking] only on the basis of suspicion, which is mere imagination and illusions.  As Allah says: Avoid much suspicion; indeed some suspicions are sins [49:12] and according to the Hadith: Beware of suspicion, for suspicion is the falsest of speech.

The following Hadith is found in Sunan Abu Dawud: What an evil habit it is for a man to say, `They claimed...'

According to another Hadith: The worst of lies is for a man to claim to have seen something that he has not seen.

In the Sahih it says: Whoever claims to have seen a dream (when he has not seen) will be told on the Day of Resurrection to make a knot between two barley grains, and he will not be able to do it.

"...each of those ones..."

Means these faculties: hearing, sight and the heart,

"...will be questioned."
Means the person will be asked about them on the Day of Resurrection, and they will be asked about him and what he did with them.

Taken from: Tafsir Ibn Kathir