Śatapatha Brāhmaṇa ("of a hundred paths") 4.3.4.7

4.3.4.7

catasro vai dakṣiṇāḥ | hiraṇyaṃ gaurvāso 'śvo na vai tadavakalpate yadaśvasya pādamavadhyādyadvā gauḥ pādamavadadhyāttasmāddaśāhomīye vāsasi hiraṇyam prabadhyāvadhāya juhoti

Now, there are four (kinds of) gifts to priests,--gold, the cow, cloth, and the horse. But it is not proper that he should lay a horse's foot or a cow's foot (into the spoon): hence he ties up a piece of gold in a fringed cloth, and having laid it (into the spoon) he offers.