(7) Fasting
كتاب الصوم
(10c) Chapter: Private Devotional Exercises in a Mosque - Section 3
(9) باب الاعتكاف - الفصل الثالث
Mishkat al-Masabih 2108 
Ibn ‘Abbas reported God’s messenger as saying about one who observes a period of private devotion in a mosque, “He turns away from sins and he is rewarded for good deeds like one who does all good deeds.” Ibn Majah transmitted it.
وَعَنِ ابْنِ عَبَّاسٍ: أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ فِي الْمُعْتَكَفِ: «هُوَ يَعْتَكِفُ الذُّنُوبَ وَيُجْرَى لَهُ مِنَ الْحَسَنَاتِ كَعَامِلِ الْحَسَنَات كلهَا» . رَوَاهُ ابْن مَاجَه
Reference : Mishkat al-Masabih 2108
In-book reference : Book 7, Hadith 148