Is staying in mosques for three days, or forty days, or four months with the intention of going out in the path of Allah considered a Sunnah of the Messenger of Allah, peace and blessings be upon him?

It is permissible to sleep in the mosque, and there is no dislike in it. This is the view of Ash-Shafi'i, Ibn Al-Musayyab, 'Ata', and Al-Hasan. Ibn Al-Mundhir permitted it, while Al-Awza'i disliked it. Malik, Ahmad, and Ishaq differentiated between a traveler and a non-traveler. Ash-Shafi'i supported the permissibility of sleeping in the mosque with what is authentically reported in the Sahihayn from Ibn 'Umar, the people of the Suffah, the 'Urayniyyin, 'Ali, Safwan ibn Umayyah, the woman with the sash, and Thumamah ibn Uthal, all of which occurred during the time of the Prophet, peace and blessings be upon him.