SB 6.7.28

putrāṇāṁ hi paro dharmaḥ
 pitṛ-śuśrūṣaṇaṁ satām
api putravatāṁ brahman
 kim uta brahmacāriṇām
Word for word: 
putrāṇām — of sons; hi — indeed; paraḥ — superior; dharmaḥ — religious principle; pitṛ-śuśrūṣaṇam — the service of the parents; satām — good; api — even; putra-vatām — of those who have sons; brahman — O dear brāhmaṇa; kim uta — what to speak; brahmacāriṇām — of brahmacārīs.
Translation: 
O brāhmaṇa, the highest duty of a son, even though he has sons of his own, is to serve his parents, and what to speak of a son who is a brahmacārī?