When someone leaves the mosque, he should greet the Prophet (ﷺ) and say:
اَللَّهُمَّ اعْصِمْنِى مِنَ الشَّيْطَانِ الرَّجِيْمِ
Pronunciation
Allaahumma aa'simni minash shaitaanir rajeem
Meaning
O Allah, protect me from Satan the outcast.
Source: Sahih (Albani). Ibn Majah: 773
Read the hadith
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، حَدَّثَنَا أَبُو بَكْرٍ الْحَنَفِيُّ، حَدَّثَنَا الضَّحَّاكُ بْنُ عُثْمَانَ، حَدَّثَنِي سَعِيدٌ الْمَقْبُرِيُّ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ـ صلى الله عليه وسلم ـ قَالَ " إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيُسَلِّمْ عَلَى النَّبِيِّ ـ صلى الله عليه وسلم ـ وَلْيَقُلِ اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ وَإِذَا خَرَجَ فَلْيُسَلِّمْ عَلَى النَّبِيِّ وَلْيَقُلِ اللَّهُمَّ اعْصِمْنِي مِنَ الشَّيْطَانِ الرَّجِيمِ " .
It was narrated from Abu Hurairah that: The Mesenger of Allah said: "When anyone of enters the mosque, let him send peace upon the Prophet, then let him say: 'Allahumma aftahli abwaba rahmatik (O Allah, open to me the gates of Your mercy).' And when he leaves, let him send peace upon the Prophet and say: 'Allahumma- simni minash-shaitanir-rajim (O Allah, protect me from the accursed Shaitan).'" [Sahih (Albani). Ibn Majah: 773]
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset