It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the people of Paradise see them, they will say: Who are these people? And it will be said: These are the 'Jahannamiyyun' (the people of Hell)."
Hadith Referenceالمعجم الأوسط / باب الألف / 2887
Hadith Takhrij«أخرجه أحمد فى «مسنده» برقم: 12442، 12464، 13095، 13885، 13886، 13948»