Śatapatha Brāhmaṇa ("of a hundred paths") 1.4.2.19

1.4.2.19

āsīno yājyāṃ yajati | iyaṃ hi yājyā tasmanna kaścana tiṣṭhanyājyāṃ yajatīyaṃ hi yājyā tadiyamevaitadbhūtvā yajati tasmādāsīnā yājyāṃ yajati

The offering-prayer he pronounces while sitting, since the offering-prayer (represents) this (earth): hence no one pronounces the offering-prayer while standing; for the offering-prayer is this earth, and by it he pronounces that which is this (earth). This is the reason why he pronounces the offering-prayer while sitting.