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?
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
पुत्राणां हि परो धर्मः पितृशुश्रूषणं सताम् । अपि पुत्रवतां ब्रह्मन् किमुत ब्रह्मचारिणाम्
putrāṇāṃ hi paro dharmaḥ pitṛśuśrūṣaṇaṃ satām | api putravatāṃ brahman kimuta brahmacāriṇām
SB 6.7.28