What he named as the least reliable speech there is
Four prohibitions, then one instruction.
إِيَّاكُمْ وَالظَّنَّ، فَإِنَّ الظَّنَّ أَكْذَبُ الْحَدِيثِ، وَلاَ تَجَسَّسُوا، وَلاَ تَحَسَّسُوا، وَلاَ تَبَاغَضُوا، وَكُونُوا إِخْوَانًا
Iyyākum wa-ẓ-ẓann, fa-inna-ẓ-ẓanna akdhabu-l-ḥadīth. Wa lā tajassasū, wa lā taḥassasū, wa lā tabāghaḍū, wa kūnū ikhwānā
Beware of assumption, for assumption is the falsest speech. Do not spy, do not pry, do not hate one another. And be brothers.
Where this comes from
Assumption is not called a bad habit or a weakness. It is put in the category of speech, and then ranked as the least true kind there is, below outright lying.
The full narration
Narrated Abu Huraira:The Prophet (ﷺ) said, "Beware of suspicion (about others), as suspicion is the falsest talk, and do not spy upon each other, and do not listen to the evil talk of the people about others' affairs, and do not have enmity with one another, but be brothers. And none should ask for the hand of a girl who is already engaged to his (Muslim) brother, but one should wait till the first suitor marries her or leaves her
Source
Ṣaḥīḥ al-Bukhārī · 5144
- Agreed upon as authentic (al-Bukhārī / Muslim)