Hadith 4:898
9764. Yazid narrated to us, he said: Al-Mas'udi informed us, from Al-Muhajir Abu Al-Hasan, from 'Ata' ibn Yasar, he said: The Messenger of Allah (may Allah bless him and grant him peace) would not fast in any month more than he fasted in Sha'ban. And that is because the lifespans of those who will die during the year are transcribed in it.
حدثنا يزيد قال أخبرنا المسعودي عن المهاجر أبي الحسن عن عطاء بن يسار قال لم يكن رسول الله صلى الله عليه وسلم في شهر أكثر صياما منه في شعبان وذلك أنه تنسخ فيه آجال من يموت في السنة