Surah Al-Baqarah, Ayat 183-184

Ayat 183. O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqun (the pious).

Ayat 184. [Observing Saum (fasts)] for a fixed number of days, but if any of you is ill or on a journey, the same number (should be made up) from other days. And as for those who can fast with difficulty, (e.g. an old man, etc.), they have (a choice either to fast or) to feed a Miskin (poor person) (for every day). But whoever does good of his own accord, it is better for him. And that you fast, it is better for you if only you know. 

Hazrat Ibn `Abbas (May Allah be pleased with them) reported: The Messenger of Allah  [SAWW](PBUH) said, "Do not observe Saum (fasting) before the advent of Ramadan. Observe Saum at sighting of the crescent of Ramadan and terminate it at sighting the crescent (of Shawwal). If the sky is overcast, complete (the month as) thirty (days).''

[At-Tirmidhi Hadith # 688].  

Lesson : as mentioned above in Surah Baqarah Ayat 184. "[Observing Saum (fasts)] for a fixed number of days,)". from this Hadith we learn "Before the advent of Ramadan'' here signifies the second half of Shawwal. That is, one should not observe voluntary Saum after 15th of Sha`ban so that one's vitality is maintained for the Saum of Ramadan approaching near. If the moon is not sighted due to clouds, then observing Saum should be started after completing thirty days of Sha`ban. Similarly, if the moon of the of Shawwal is not sighted, then thirty days of fasting Ramadan must be completed before celebrating `Eid-ul-Fitr.