Reference: Fataawa Ramadhaan – Volume 2, Page 744, Fatwa No.755
Fataawa ash-Shaykh Muhammad as-Saalih al-Uthaymeen – Volume 1, Page 562
Question: I have heard there are [different] categories in fasting, what is the truth of this statement?
And is there a specific reward for each?
Response: By categories, if you mean obligatory (fardh) and voluntary (nafl), then this is correct. The obligatory is better than the voluntary. As for the degree of excellence and reward, then the reward is with Allaah. With respect to those fasting, then here there is a big difference according to what the person does whilst fasting with respect to adhering to good Islaamic manners and behaviour and lack of doing so. Also, according to that which he has in his heart in terms of sincerity (ikhlaas).