Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 12
Sanskrit:
अयने चाहनी प्राहुर्वत्सरो द्वादश स्मृत: ।संवत्सरशतं नृणांपरमायुर्निरूपितम् ॥ १२ ॥
ITRANS:
ayane cāhanī prāhurvatsaro dvādaśa smṛtaḥsaṁvatsara-śataṁ nṝṇāṁparamāyur nirūpitam
Translation:
Two solar movements make one day and night of the demigods, and that combination of day and night is one complete calendar year for the human being. The human being has a duration of life of one hundred years.