If his garment is impure and he wants to pray: if a quarter of his garment or more is pure, he can pray in the impure garment, and it is not permissible for him to pray naked; because the quarter has the ruling of the whole, and as long as a quarter of his garment is pure, he must pray in it. However, if the pure part of the garment is less than a quarter, it is preferable for him to pray in it, and he is allowed to pray naked; because his situation is like that of someone who has no garment. See: Al-Wiqayah and its explanation by Sadr al-Shari'ah 1: 143.