It was said to the Prophet ﷺ, "The left side of the mosque has been left empty."The Prophet ﷺ replied, "Whoever fills the left side of the mosque, a double share of reward will be recorded for him."
حدثنا محمد بن أبي الحسين أبو جعفر حدثنا عمرو بن عثمان الكلابي حدثنا عبيد الله بن عمرو الرقي عن ليث بن أبي سليم عن نافع
قيل للنبي ﷺ إن ميسرة المسجد تعطلت فقال النبي ﷺ من عمر ميسرة المسجد كتب له كفلان من الأجر
No references are available.
[Its chain of transmission is daif] Tarsusi collected it in Musnad Ibn Umar, no. 95, from the hadith of Amr bin Uthman, through this route. See no. 208 for its defect. Busiri said, "This chain is daif due to the weakness of Layth bin Abu Salim."