Gobhilagrihyasutra 3.7.21

3.7.21

uttarato 'gner darbhastambaṃ samūlaṃ pratiṣṭhāpya somo rāje 'ty etaṃ mantraṃ japati, yāṃ sandhāṃ sam adhatte 'ti ca.

To the north of the fire he places a bunch of Darbha grass with roots, and murmurs the Mantra, 'Soma the king' (ibid. 4), and, 'The agreement which you have made' (ibid. 5).