Kauṣītaki (Śāṅkhāyana) Brāhmaṇa 3.8.6

3.8.6

atha yad asamnayan puroḍāśāv antareṇa upāṃśv ājyasya yajaty ajāmitāyai / (DpM)

Now, as to why, when not offering the sānnāyya, he sacrifices with clarified butter in a whisper between the two sacrificial cakes, [it is] to prevent monotony.