When leaving the mosque, step out with the left foot and say: 'Bismillah, wassalatu wassalamu 'ala Rasulillah. Allahumma inni as'aluka min fadlik. Allahumma'simni minash-shaytanir-rajim.' This asks Allah for His bounty as you re-enter the world.
بِسْمِ اللَّهِ، وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُولِ اللَّهِ. اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ. اللَّهُمَّ اعْصِمْنِي مِنَ الشَّيْطَانِ الرَّجِيمِ
Bismillah, wassalatu wassalamu 'ala Rasulillah. Allahumma inni as'aluka min fadlik. Allahumma'simni minash-shaytanir-rajim.
Meaning: In the name of Allah, and blessings and peace be upon the Messenger of Allah. O Allah, I ask You from Your bounty. O Allah, protect me from the accursed Shaytan.
Source: Sahih Muslim #713The wisdom of asking for Allah's bounty when leaving (rather than mercy, which is asked when entering) is that inside the mosque one is engaged in worship and needs mercy, while upon leaving one returns to worldly activities and needs Allah's bounty and provision. The Shaytan waits at the doors of the mosques, so seeking protection from him upon exit is vital.
بِسْمِ اللَّهِ، وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُولِ اللَّهِ. اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ. اللَّهُمَّ اعْصِمْنِي مِنَ الشَّيْطَانِ الرَّجِيمِ
The Prophet (peace be upon him) said: 'When one of you enters the mosque, let him send salah upon the Prophet and say: Allahumma iftah li abwaba rahmatik. And when he leaves, let him send salah upon the Prophet and say: Allahumma inni as'aluka min fadlik.'