اَلۡـحَـمۡـدُ   لِـلّٰـہِ  رَبِّ  الۡـعٰـلَـمِیۡنَ   وَ  الـصَّـلٰـوۃُ   وَ  الـسَّـلَامُ  عَـلٰی   خَاتَمِ النَّبِیّٖن

اَمَّـا  بَــعۡـدُ   فَـاَعُـوۡذُ   بِـا لـلّٰـہِ   مِـنَ   الـشَّـیۡـطٰنِ   الـرَّجِیۡمِؕ    بِـسۡمِ   الـلّٰـہِ   الـرَّحۡـمٰنِ   الـرَّحِـیۡمِؕ

One of the purposes behind the revelation of the noble Quran

اَلصَّلٰوۃُ وَ السَّلَامُ عَلَیْكَ یَا رَسُولَ اللہ                                                                                    وَعَلٰی اٰلِكَ وَ اَصْحٰبِكَ یَا حَبِیْبَ اللہ

اَلصَّلٰوۃُ وَ السَّلَامُ عَلَیْكَ یَا نَبِیَّ اللہ                                                                                                     وَعَلٰی اٰلِكَ وَ اَصْحٰبِكَ یَا نُوْرَ اللہ

نَوَيۡتُ سُنَّتَ الۡاِعۡتِكَافِ

Translation: I have made the intention of Sunnah iꜤtikāf.

Whenever you enter a masjid, upon remembering, make the intention of iꜤtikāf  because as long as you stay in the masjid you will keep getting the reward of iꜤtikāf. Remember! There is no SharꜤī permission to eat and drink, sleep or do suḥūr and ifṭārī even to drink Zamzam water and the water on which dam has been made in a masjid. However, if the intention of iꜤtikāf  is made, all these acts will become permissible. One should not make intention to observe iꜤtikāf only to eat, drink or sleep, but rather he should do it for pleasing Allah Almighty.

It is stated in Fatāwā Shāmī:

If someone wants to eat, drink or sleep in a masjid, he should make intention to observe iꜤtikāf, do dhikr of Allah Almighty for some time, then he can do whatever he wants (i.e., now if he wants to eat, drink or sleep, he can do so.).