If you read Solusi Ed.34, on the first page you will find a section for Hadis Qudsi. For this month, the Hadis Qudsi reads as follows:
Daripada Abu Hurairah r.a, Rasulallah s.a.w bersabda: "Allah berfirman: "Setiap amalan manusia itu untuknya kecuali puasa. Ia untuk-Ku dan Aku yang memberi ganjarannya"".
(Riwayat al-Bukhari dan Muslim)
The last religious class that we had with Ustaz Firdaus, he explained the hadith further. According to ustaz, there are some arguments amongst the ulama' over this hadith. It is because, literally, all ibadah are rewarded by Allah and only He knows how and how much the reward would be. Therefore, what makes it different with AS-Saum, fast?
And finally, jumhur ulama' (jumhur means the group of renowned ulama') agrees that what makes fast or as-saum different is because it is an ibadah that no other people would know whether you are dong it or not, and only Allah knows. This is very different with any other ibadah like solat, zakat and performing hajj. All this other ibadah can be seen by human eyes whenever one performs it. Everybody can see you performing solat (especially if in jamaah), everybody can know that you are performing hajj (in fact almost whole Malaya knows about it when one perfoms hajj), so does when someone pay the zakat. For zakat, at least one person will know that you are performing zakat that is the amil (the zakat collector).
For as-saum, people will not know if you are fasting unless you tell them that you are fasting (exceptional for the month of Ramadhan). An ibadah that no one knows, and you sincerely perform it, is a great ibadah in the eyes of Allah.
So, everybody, if you are fasting (sunat), please try as best as possible not to tell others, try to find excuses (but not to lie) when people invite you for lunch etc. But in certain circumstances, it's ok to tell people that you're fasting. You must know what is best especially in order to protect yourself from riya' and takabbur.