Hadith: You praised this, so Paradise has been affirmed for him, and you dispraised this, so Hell has been affirmed for him. You are Allah's witnesses on earth

عن أنس رضي الله عنه قال: مَرُّوا بجَنَازَةٍ، فأَثْنَوْا عليها خيراً، فقال النبي صلى الله عليه وسلم : «وَجَبَتْ» ثم مَرُّوا بأخرى، فأَثْنَوْا عليها شراً، فقال النبي صلى الله عليه وسلم : «وَجَبَتْ»، فقال عمر بن الخطاب رضي الله عنه : ما وَجَبَتْ؟ فقال: «هذا أَثْنَيْتُمْ عليه خيراً، فوَجَبَتْ له الجنة، وهذا أَثْنَيْتُم عليه شراً، فوَجَبَتْ له النار، أنتم شُهَدَاءُ الله في الأرض».
[صحيح] - [متفق عليه]

Anas ibn Mālik (may Allah be pleased with him) reported: They (the Companions) passed by a funeral procession and praised the deceased. The Prophet (may Allah's peace and blessings be upon him) said: "It has been affirmed." Then they passed by another funeral procession but dispraised the deceased, so the Prophet (may Allah's peace and blessings be upon him) said: "It has been affirmed." ‘Umar ibn al-Khattāb (may Allah be pleased with him) asked: "What has been affirmed?" He replied: "You praised this, so Paradise has been affirmed for him, and you dispraised this, so Hell has been affirmed for him. You are Allah's witnesses on earth."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

Some of the Companions passed by a funeral procession and testified to the good nature and righteousness of the deceased and his upright following of the Shariah of Allah. Having heard their praise of him, the Prophet (may Allah's peace and blessings be upon him) commented that it was affirmed. Then they passed by another funeral procession and testified to the evil of the deceased. Thereupon, the Prophet (may Allah's peace and blessings be upon him) commented that it was affirmed. ‘Umar ibn al-Khattāb (may Allah be pleased with him) asked about the meaning of "it has been affirmed", as mentioned in both comments. The Prophet (may Allah's peace and blessings be upon him) explained that Paradise has been affirmed for the one regarding whom you attested to his good nature, righteousness, and uprightness and that Hell is affirmed for the one regarding whom you attested to his evil. It could be that this man was known to be a hypocrite or something like that. Then, the Prophet (may Allah's peace and blessings be upon him) informed that if people of truthfulness, virtue, and righteousness testify that someone deserves Paradise or deserves Hell, then this will be the case in reality.