She said, "The Messenger of Allah ﷺ used to fast until we would say he never breaks his fast, and he would go without fasting until we would say he never fasts. I never saw the Messenger of Allah ﷺ complete a full month of fasting except Ramadan, and I never saw him fast more in any month than he did in Shaban."
دثنا عبد الله بن مسلمة عن مالك عن أبي النضر مولى عمر بن عبيد الله عن أبي سلمة بن عبد الرحمن
أنها قالت كان رسول الله ﷺ يصوم حتى نقول لا يفطر ويفطر حتى نقول لا يصوم وما رأيت رسول الله ﷺ استكمل صيام شهر قط إلا رمضان وما رأيته في شهر أكثر صياما منه في شعبان
Bukhari narrated it in Kitab al-Sawm, Chapter on Fasting in Shaban, no. 1969, and Muslim in Kitab al-Siyam, Chapter etc., no. 1156, from the hadith of Malik through this chain, and it is found in Muwatta under the Prophet's ﷺ fasting outside of Ramadan (Yahya).