Is it obligatory to repeat the prayer if there is a possibility that the Imam's clothes are ritually impure?

Prayer is valid in a dirty garment if it is pure (tahir), and its impurity (najasa) is neither certain nor highly probable. Mere doubt about the impurity of the Imam's clothes does not prevent praying behind him. It is preferable for the Imam, and for any worshipper generally, to come to prayer in their best appearance and finest clothes, as Allah Almighty says: "O Children of Adam, take your adornment at every masjid." As for a garment with sweat or non-impure dirt, or one that is old and covers the 'awrah, there is no harm in praying in it.