Manava Dharmashastra 1.108

1.108

paramaḥ prakṛṣṭo dharma ācārastathā śrutau vede ya uktaḥ, smārtaḥ smṛtiṣūktastasmādācāradharme nityaṃ yuktaḥ syānnityamanutiṣṭhedātmavānātmano hitamicchan | sarvasyā''tmāstyato matupā taddhitaparatvamucyate |

Morality [Right Behaviour] is highest Dharma; that which is prescribed in the śruti and laid down in the Smṛti. hence the twice-born person, desiring the welfare of his soul, should be always intent upon Right Behaviour.—(108)

Medhātithi’s commentary (manubhāṣya):
‘Highest Dharma is Morality’ [Right Behaviour]; that which has been prescribed in the Śruti, i.e. Veda; and also ‘that laid down in the Smṛti.’ Hence one should be ever intent upon Dharma in the shape of Right Behaviour i.e. he should carry it into practice.

‘Ātmavān’—lit. ‘endowed with soul,’ really means ‘desiring the welfare of his soul’; all men are ‘endowed with soul’; hence the affix ‘matup’ is taken to mean ‘welfare of soul.’—(108).