Sufyaan Thowri (Rahimahullah) was among the famous Imaams in the science of Hadeeth and Islamic law. He once mentioned that he was guilty of a sin due to which he was deprived of offering Tahajjud Salaah for five months. When someone asked him as to what it was, he replied that he had once seen a man crying and thought that the man was a hypocrite.
Imagine the evil consequence of a sinful thought! Yet we say harsh things about people without even thinking twice. And, Allah forbid, if we have something against someone, we cross all limits in slandering him. We even turn his virtues into vices, and publicise and magnify his weaknesses if any. (Fazaail-e-Sadaqaat, p. 74)