What is the ruling on praying behind an imam who stutters, despite his beautiful voice, good recitation, and knowledge of the rules of Tajwid?

The prayer leadership (imamah) of someone who substitutes one letter for another is not valid, unless he leads someone similar to him in prayer. This applies if the error is in Al-Fatiha. However, if the error is in something other than Al-Fatiha, then praying behind him is valid, unless he did so intentionally. As for someone who omits a letter from Al-Fatiha due to inability, or substitutes it with another, or makes an error that changes the meaning, and is unable to correct it, he is like an illiterate person. It is not permissible for a reciter to be led in prayer by him, but it is permissible for each of them to lead someone similar to himself.