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 a quarter has the same ruling as the whole, and as long as a quarter of his garment is pure, he must pray in it. This is different from the case where the pure part of the garment is less than a quarter, in which case it is preferable for him to pray in it, and it is permissible for him 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.