Śatapatha Brāhmaṇa ("of a hundred paths") 11.4.1.6

11.4.1.6

sa vai gautamasya putra vṛto janaṃ dhāvayet yastaddarśapūrṇamāsayorvidyādyasmādimāḥ prajā lomaśā jāyante yasmādāsām punariva śmaśrūṇyaupapakṣyāṇi durbīriṇāni jāyante yasmācīrṣaṇyevāgre palito bhavatyatha punaruttame vayasi sarva eva palito bhavati

'He alone, O son of Gautama, may drive about amongst people as chosen (priest), who knows from the Full and New-moon sacrifices, whereby creatures here are born with hair; whereby, for the second time, as it were, the hair of the beard and the arm-pits and other parts of the body grow on them; whereby it is on the head that one first becomes grey, and then, again, in the last stage of life, one becomes grey all over.