Prohibition of fasting on the day of doubt
The day of doubt is the thirtieth of Sha’baan if the new moon cannot be seen because of clouds, etc. It is called the day of doubt because it could be the thirtieth of Sha’baan, or it could be the first day of Ramadaan. It is haraam to fast on this day, unless it coincides with a day on which a person habitually fasts.
Narrated Abdullah bin Umar (R.A.) that Allah’s Apostle (peace and blessings of Allah be upon him) mentioned Ramadan and said, ”Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha’ban as 30 days).” [Bukhari vol 3, bk 31, no 130]
Al-Tirmidhi (686) and al-Nasaa’i (2188) narrated that ‘Ammaar ibn Yaasir (may Allaah be pleased with him) said: “Whoever fasts on the day concerning which there is doubt has disobeyed Abu’l-Qaasim (peace and blessings of Allah be upon him).”
Al-Haafiz said in Fath al-Baari: It was understood from this that it is haraam to fast the day of doubt, because the Sahaabah would not say such a thing based on personal opinion.
Making Intention is Obligatory
It is essential to make the intention to fast the month of Ramadan at night, before Fajr. It is not sufficient to start fasting that day without the intention. Whoever finds out at the time of Duha that today it is Ramadan and makes the intention of fasting has to refrain from eating until sunset, and he also has to make up that day, because it was narrated by Ibn ‘Umar from Hafsah (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him) said: “Whoever does not have the intention of fasting before Fajr, there is no fast for him.” Narrated by Imam Ahmad, the authors of al-Sunan, Ibn Khuzaymah and Ibn Hibbaan; they classed it as saheeh and marfoo’.
Culled from http://www.islamqa.com
Wishing You Ramadan Mubarak in Advance!!!