Mahdi bin Maymun narrates: I heard al-Hasan bin Dinar ask al-Hasan: "Which verse is the hardest upon the people of Hell?" He said: "I asked Sayyiduna Abu Barzah (may Allah be pleased with him) this question, and he said: 'The hardest verse that has been revealed is this: "So taste (the punishment)! We will not increase you except in punishment."'"
Tabarani recorded it; its chain contains a narrator, Shu'ayb bin Bayan, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب التفسير / 11463