Kaushitakagrihyasutra 2.8.6

2.8.6

tvamagne vratabhṛcchuciḥ ityetābhyāmṛgbhyāmāhutīrhutvā atha mekhalāṃ śānte vṛkṣe nidhāya pūrṇe kāle mekhalāmupavītaṃ ca daṇḍe badhnāti \6\

Having offered oblations with the two verses, “Thou, Agni, vow-holder, pure…”, then placing the girdle on an auspicious tree, when the time is complete, he ties the girdle and the sacred thread to the staff.