عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ يَتَحَفَّظُ مِنْ هِلَالِ شَعْبَانَ مَا لَا يَتَحَفَّظُ مِنْ غَيْرِهِ، ثُمَّ يَصُومُ بِرُؤْيَةِ رَمَضَانَ، فَإِنَّ غُمَّ عَلَيْهِ عَدَّ ثَلَاثِينَ يَوْمًا ثُمَّ صَامَ
It is narrated from Sayyida Aisha (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) used to pay more attention to the crescent of Sha'ban than to the crescent of any other month. Then, when the crescent of Ramadan was sighted, he would begin fasting, and if the sky was cloudy, he would complete the count of thirty days of (Sha'ban), and then begin fasting.