Is there some dua to make niyat for fardh sawm (in Ramadan) or nafl sawm?
بسم الله الرحمن الرحيم
(Fatwa: 712/677/SN=10/1436)
No, no special dua is recorded in Hadiths about it. Just one should make intention in his heart that he shall keep fast tomorrow. It is enough.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband