Hiranyakeshigrihyasutra 2.1.4.17

2.1.4.17

paśūnāṃ tvā huṃkāreṇābhijighrāmyasāvāyuṣe varcase hum \ iti mūrdhnyabhijighrya \17\

With (the formula), 'With the humkâra (the mystical syllable hum) of the cattle I kiss thee, N.N.! For the sake of long life and of glory! Hum!' he kisses his head. Then he seizes with his right hand (his son's) right hand together with the thumb, with the five sections, 'Agni is long-lived.'