Manava Dharmashastra 9.102

9.102

tathā nityaṃ yateyātāṃ strīpuṃsau tu kṛtakriyau
yathā tau viyuktāv itaretaram //

Let man and woman, united in marriage, constantly exert themselves, that (they may not be) disunited (and) may not violate their mutual fidelity.