If some hadiths are combined, the Dua for going out the mosque will be as follows:

بِسْمِ اللَّهِ اَللَّهُمَّ صَلِّ عَلَى مُحَمَّدٍ، اَللَّهُمَّ إِنِّيْ أَسْأَلُكَ مِنْ فَضْلِكَ

Bismillahi Allaahumma salli 'ala muhammad, Allaahumma inni as-aluka min fadlik

In the name of Allah (I am going out).Oh Allah! Please send bessings and peace upon muhammad (ﷺ). Oh Allah! I want from your bounty.

Reference: Hasan (Albani). Ibn Sunni: 88, Muslim: 713