Riyad as-Salihin (The Meadows of the Righteous) by Imam Nawawi, chapter 225: Clarification of the excellence of fasting Muharram, Sha’ban and the sacred months
Hadith 1247. ‘A’isha (radiAllaho anha) said, “The Prophet, may Allah bless him and grant him peace, did not fast in any month more than Sha’ban. He used to fast all of Sha’ban.” In one variant, “He used to fast Sha’ban except for a little.” [Agreed upon]