Kautilya's Arthashastra 9.2.21

9.2.21

"brāhmaṇakṣatriyavaiśyaśūdrasainyānāṃ tejaḥprādhānyāt pūrvaṃ pūrvaṃ śreyaḥ samnāhayitum" ity ācāryāḥ //

My teacher says that of the armies composed of Bráhmans, Kshatriyas, Vaisyas, or Súdras, that which is mentioned first is, on account of bravery, better to be enlisted than the one subsequently mentioned in the order of enumeration.