Is it permissible to adopt the saying: "None shall lead you in prayer but a man of sound build," and is it a Hadith, and is it valid for a blind person to lead the prayer, even though he might be the most knowledgeable in the Qur'an in the mosque?

The mentioned narration is not found in the Sunnah collections. There is no objection to a blind person leading the prayer if he is the best reciter among those present and the most knowledgeable of the jurisprudence of prayer. The jurists have agreed on the validity of a blind person's imamate, and they cited that the Prophet, peace and blessings be upon him, appointed Ibn Umm Maktum to lead the people in prayer while he was blind. Scholars differed regarding preference when there is equality in merit; the Shafi'is hold that they are equal, while the Hanafis, Malikis, and Hanbalis prefer the sighted person over the blind.