What is the ruling on the prayer of one whose garment trails on the ground (musbil), and praying behind him, and behind an innovator (mubtadi')?

Trailing one's garment (isbal) is reprehensible in all circumstances, and forbidden if done out of conceit, based on the Prophet's (peace be upon him) saying: "Whatever of the izar (lower garment) is below the ankles is in the Fire." The prayer of one who trails his garment is considered valid by the majority of scholars, though it is reprehensible to the point of being forbidden if done out of conceit. The hadith (definition) in Abu Dawud regarding the non-acceptance of the prayer of one who trails his garment is weak. Even if we assume its authenticity, non-acceptance does not mean invalidity; rather, it negates the attainment of reward. This is because validity is established for one who consults a soothsayer or drinks alcohol, yet is not commanded to repeat their prayers. Therefore, the preponderant view is the validity of the prayer of one who trails his garment, and the validity of praying behind him.