Musnad Ahmad Hadith 24116

Abu Salamah (may Allah have mercy on him) says that I asked Aisha (may Allah be pleased with her), "Mother! Tell me something about the prayer of the Prophet (ﷺ)." She replied, "The (tahajjud) prayer of the Prophet (ﷺ) was the same in Ramadan and outside of Ramadan, that is, thirteen rak‘ahs, which included the two sunnah of Fajr." I asked, "Now tell me about his voluntary fasts." She replied, "Sometimes the Prophet (ﷺ) would fast so much that we would say, 'Now the Prophet (ﷺ) will keep fasting continuously,' and sometimes he would leave fasting for so many days that we would say, 'Now the Prophet (ﷺ) will keep leaving fasting.' And I have not seen the Prophet (ﷺ) fast in any month as much as I saw him fast in Sha‘ban. The Prophet (ﷺ) would fast almost the entire month, leaving only a few days."