Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 11
Sanskrit:
एतत् कल्पविकल्पस्य प्रमाणं परिकीर्तितम् ।यत्र मन्वन्तराण्याहुश्चतुर्दश पुराविद: ॥ ११ ॥
ITRANS:
etat kalpa-vikalpasyapramāṇaṁ parikīrtitamyatra manvantarāṇy āhuścaturdaśa purāvidaḥ
Translation:
In one kalpa, or one day of Brahmā, there take place the many changes called vikalpas. O King, all of these have been previously described to you by me. Learned scholars who know the past, present and future have ascertained that in one day of Brahmā there are fourteen Manus.