Manava Dharmashastra 1.79

1.79

ekasaptatidaivikāni yugāni manvantaraṃ nāma kālaḥ |

The ‘Time-cycle of the Gods’ which has been described above as consisting of ‘twelve thousand periods,’—this multiplied by ‘seventy-one’ forms what is known here as ‘Manvantara’ (Regime of a Manu).—(79)

Medhātithi’s commentary (manubhāṣya):
The period of time named ‘manvantara’ consists of seventy-one ‘Time-cycles of the gods.’—(79)