What is the ruling on a prayer in which part of the Imam's back was exposed during prostration due to the shortness of his shirt?

If a part of the back, not facing below the navel, becomes uncovered, the prayer is not invalidated. If it does face below the navel, it is still not invalidated according to the Malikis and Hanafis, as it is considered a light (or minor) 'awrah. If the uncovering is slight (minimal), the prayer is not invalidated; this is the view of Ahmad and Abu Hanifa, who cite the Hadith of Amr ibn Salama al-Jarmi as evidence. "Slight" refers to what is not excessive according to custom. The prayer of the congregants remains valid even if they look at what has been uncovered from the 'awrah.