Shatapatha Brahmana 1.9.2.3

1.9.2.3

jaghaneno haiva patnīṃ | ekeṣāmadhvaryureti no eva tathā kuryātpūrvārdho vai yajñasyādhvaryurjaghanārdhaḥ patnī yathā bhasattaḥ śiraḥ pratidadhyādevaṃ tadbahirdhā haiva yajñātsyādyatteneyāt

According to others, the Adhvaryu walks (so as to pass) behind the (sacrificer's) wife. Let him not, however, do this either; for verily the Adhvaryu is the fore-part, and the wife is the hind-part of the sacrifice: hence, if he were to pass so, it would be as if one were to put his head behind; and he (the Adhvaryu) would be outside the sacrifice.