Ash-Shafi’ee: The Sunnah is to seek seclusion in the mosque during the last ten nights of Ramadan

Imam Ash-Shafi’ee and his companions, may Allah have mercy on them, said, “Whoever desires to follow the example of the Prophet, peace be upon him, should seek seclusion in the mosque during the last ten nights of Ramadan; so let him enter the mosque before the setting of the sun on the twenty first night.”

[An-Nawawi, Al-Majmu 501]

قال الشافعي والأصحاب ومن أراد الاقتداء بالنبي صلى الله عليه وسلم في الاعتكاف في العشر الأواخر من رمضان فينبغي أن يدخل المسجد قبل غروب الشمس ليلة الحادي والعشرين منه

المجموع شرح المهذب يحيى بن شرف النووي