Is a Muslim's saying to his brother, "Your reckoning will be difficult on the Day of Resurrection," considered an usurpation of God's authority, and what is the ruling on causing his brother grief and pain with his words?

It is not permissible for a Muslim to say to his Muslim brother: "Your reckoning will be difficult on the Day of Judgment," or similar phrases. This is because the matter of reckoning is unseen, known only to Allah, and it involves presuming to speak for Allah and uttering statements about Him without knowledge. This resembles "making a presumption about Allah," which nullifies the deeds of its speaker. Furthermore, it involves harming and frightening a Muslim, and the Prophet, peace and blessings be upon him, forbade frightening a Muslim, whether in jest or seriously.