Abu Dawud
Fasting
Book Number: 007 / Hadith Number: 2318
Narated By 'Aisha, Ummul Mu'minin : The Apostle of Allah (pbuh) used to count the days in Sha'ban in a manner he did not count any other month; then he fasted when he sighted the new moon of Ramadan; but if the weather was cloudy he counted thirty days and then fasted.