What is the reason behind Ibn Mas'ud's statement, "On the Day of Resurrection, Allah will forgive a forgiveness that has not crossed the heart of any human," and has it been mentioned what sin Allah will forgive?

No reason was found for Ibn Mas'ud's statement. Ibn al-Mubarak and Ibn Abi al-Dunya narrated it with the wording: "Allah Almighty will forgive on the Day of Resurrection a forgiveness that has not occurred to the heart of any human." This narration indicates the vastness of Allah's mercy and forgiveness for every sin except polytheism, if one does not repent from it. It was also narrated as مرفوع (attributed to the Prophet) from Hudhayfah (may Allah be pleased with him) with a similar wording, adding: "Allah will forgive on the Day of Resurrection a forgiveness that has not occurred to the heart of any human."