SB 6.7.28
putranam hi paro dharmah
pitr-susrusanam satam
api putravatam brahman
kim uta brahmacarinam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O brahmana, 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 brahmacari?