Shatapatha Brahmana 13.5.1.3

13.5.1.3

ekaviṃśatiḥ savanīyāḥ paśavaḥ sarva āgneyāsteṣāṃ samānaṃ karmetyu haika āhurdve tvevaite ekādaśinyāvālabheta ya evaikādaśineṣu kāmastasya kāmasyāptyai

'There are twenty-one Savanîya victims, all of them sacred to Agni: for these there is one and the same performance,' so say some; but, indeed, he should immolate two sets of eleven (victims), with the view of his obtainment of whatever desired object there may be in (victims) belonging to a set of eleven.