Sayyidah Aisha Siddiqa (may Allah be pleased with her) narrates: The Prophet Muhammad (peace and blessings be upon him) used to pay more attention to sighting the crescent of Sha'ban than to anything else. Then, upon sighting the crescent of Ramadan, he would begin to fast. If it was cloudy for the Prophet (peace and blessings be upon him), he would complete the count of thirty (30) days first, and then begin to fast.
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3444
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (2014)، «الإرواء» (3/ 7 - 8)، «المشكاة» (1980).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم