In this Hadīth, Jābir (may Allah be pleased with him) says that the Prophet (may Allah's peace and blessings be upon him) clarified how grave a sin it is to swear a false oath at the Prophet's pulpit in his mosque, even concerning a trivial matter. Indeed, in this place, the Prophet (may Allah's peace and blessings be upon him) is to be glorified, and what he said on the pulpit is to be remembered. So, going against his teachings by taking a false oath makes one deserving of this threat. A false oath entails the wrath of Allah, but in such an honorable place (as the Prophet's pulpit inside his mosque) it is an uglier sin.