Shatapatha Brahmana 4.3.5.23

4.3.5.23

tadyatpratiprasthātā saṃsravau sampragṛhṇāti | ādityagraho vā eṣa bhavati na vā ādityagrahasyānuvaṣaṭkarotyetasmādvai sāvitraṃ grahaṃ grahīṣyanbhavati tadasya sāvitreṇaivānuvaṣaṭkṛto bhavati

And as to why the Pratiprasthâtri holds out the two residues: this is (the remains of) the Âditya libation, and for the Âditya libation he pronounces no Anuvashat; and from that (Âgrayana graha) he intends to draw the Sâvitra graha,--so that the Anuvashat is performed for it by means of the Sâvitra graha.