كَمْ مِنْ صَائِمٍ لَيْسَ لَهُ مِنْ صِيَامِهِ إِلَّا الظَّمَأُ، وَكَمْ مِنْ قَائِمٍ لَيْسَ لَهُ مِنْ قِيَامِهِ إِلَّا السَّهَرُ (Darimi)
Translation: Many people who fast gain nothing from their fast except thirst, and many who stand in night prayer gain nothing except sleeplessness.
صَائِمٍ — one fasting; قَائِمٍ — one standing in night worship; الظَّمَأُ — thirst; السَّهَرُ — staying awake at night.
Brief grammar: كَمْ مِنْ صَائِمٍ uses كَمْ to indicate “how many/many.” إِلَّا indicates exception: “except.”
The warning concerns the moral and spiritual fruit of worship. It should not automatically be treated as a technical ruling that every such fast or prayer is legally invalid.