Śatapatha Brāhmaṇa ("of a hundred paths") 3.4.3.19

3.4.3.19

atha prastare nihnuvate | uttarataupacāro vai yajño 'thaitaddakṣiṇevānvityāpyāyayantyagnirvai yajñastadyajñam pṛṣṭhataḥ kurvanti tanmithyākurvanti devebhya āvṛścyante yajño vai prastarastadyajñam punarārabhante tasyo haiṣā prāyaścittistatho haiṣāmetanna mithyākṛtam bhavati na devebhya āvṛścyante tasmātprastare nihnuvate

Thereupon they make amends on the prastara. For the sacrifice requires a northward attendance; but now they strengthen (Soma) after going, as it were, towards the right (south); and, the sacrifice being the fire, they thereby turn their back on the sacrifice and thus do wrong and are cut off from the gods. Now the prastara also is (part of) the sacrifice, and by (touching) it they again get hold of the sacrifice. And this is his expiation of that (transgression); and so no wrong is committed by them and they are not cut off from the gods: for this reason they make amends on the prastara.