Reference: Fataawa Ramadhaan – Volume 1, Page 161, Fatwa No.93
Majmoo’ Fataawa Shaykh al-Islaam Ibn Taymiyyah – Volume 25, Page 215
Question: What do you say about the one fasting in Ramadhaan; Is he required to make [a new] intention every day?
Response: Everyone who knows that tomorrow is a day of Ramadhaan, and he wants to fast it, then [by wanting to fast it] he has intended the fast. This is the same whether he has verbally uttered the intention or not. This is the practice of the common Muslims, all of them make the intention to fast.