Dear noble brothers, Siyaam is sincerity act of worship. Man sometimes doubts his sincerity, but he confirms it upon fasting in Ramadan, especially in a hot summer's day.

On such a day the temperature is about 44 or 45 degrees, and man is very thirsty, but he refrains himself upon entering his house from drinking cold water from the refrigerator, given no one sees him. 

What prevents him from putting even one drop of water in his mouth? His belief in Allah does. As if Allah is saying to you when you fast: "You love Me, and you believe in Me, in My Knowledge and in My Existence". 

What prevents you from drinking a glass of cold water from the refrigerator while you are alone at home and the windows are closed? You cannot do it (if you are a true believer). Thus they say, "Siyaam is sincerity act of worship."