Sunan Ibn Majah, Chapter: Chapters Regarding Funerals

"A funeral (procession) passed by The Prophet (saw) and they praised (the deceased) and spoke well of him. He said: '(Paradise is) guaranteed for him.' Then another funeral passed by and they spoke badly of him, and he (The Prophet (saw)) said: '(Hell is) guaranteed for him.' It was said: 'O Messenger of Allah, you said that (Paradise was) guaranteed for this one and that (Hell was) guaranteed for the other one.' He said: 'It is the testimony of the people, and the believers are the witnesses of Allah on earth.'" Sahih