English Translation of Hadith
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."