Shatapatha Brahmana 5.2.1.13

5.2.1.13

tadyadgodhūmānupaspṛśati | annaṃ vai godhūmā annaṃ vā eṣa ujjayati yo vājapeyena yajate 'nnapeyaṃ ha vai nāmaitadyadvājapeyaṃ tadyadevaitadannamudajaiṣīttenaivaitadetāṃ gatiṃ gatvā saṃspṛśate tadātmankurute tasmādgodhūmānupaspṛśati

And as to why he touches the wheat: wheat is food, and he who offers the Vâgapeya, wins food, for vâga-peya is the same as anna-peya (food and drink): thus whatever food he has thereby won, therewith now that he has gone to that supreme goal, he puts himself in contact, and possesses himself of it,--therefore he touches the wheat (top-piece).