Is the story of "Salam the Interpreter" – which tells of Amir al-Mu'minin sending him to inspect the dam of Gog and Magog – authentic or an innovation, and what are its details if it is authentic?

This report, circulated by historians from Ibn Khurradadhbih's book "Kitab al-Masalik wa al-Mamalik," regarding Al-Wathiq's dispatch of Sallam the Interpreter to the barrier of Gog and Magog, has an unreliable chain of transmission (isnad). Ibn Khurradadhbih's trustworthiness has been questioned, and some scholars have accused him of lying. Sallam the Interpreter is an unknown figure. Moreover, the report itself is unusual, given that no other scholar present in the caliphs' assemblies mentioned this extraordinary journey except for Ibn Khurradadhbih, who reported it from Sallam.