Source · al-Bukhārī 603; Muslim 379

Each call is made twice; the listener replies لَا حَوْلَ وَلَا قُوَّةَ إِلَّا بِاللّٰهِ instead of repeating. حَيَّ is a 'verb-noun' (اِسْم فِعْل) meaning 'hurry to'; اَلْفَلَاح shares its root with فَلَّاح 'farmer', the one who splits open the soil.