It is Sunnah of Prophet (S.A.W.) to spend the last ten days and nights of Ramadan in the Mosque. This is called Itakaf or Seclusion in Masjid. According to Hadith narrated by Abdullah bin Umar (R.A.) that:
“Allah's Messenger (S.A.W.) used to practice I`tikaf in the last ten days of the month of Ramadan.”
(Sahih Bukhari: 2025)