Khadiragrihyasutra 4.1.12

4.1.12

ardhamāsavratī paurṇamāsyāṃ rātrau nābhimātraṃ pragāhyāvidāsini hrade 'kṣatataṇḍulānāsyena juhuyādudake vṛkṣa iveti pañcabhiḥ pārthivaṃ karma / 12

Having kept the vow (of fasting) through one fortnight, he should in the full-moon night plunge up to his navel into a pool which does not dry up, and should sacrifice with his mouth fried grain into the water, with the five (verses), 'Like a tree' (MB. II, 4, 9-13).