سُبْحَانَ اللَّهِ وَالْحَمْدُ لِلَّهِ وَلَا إِلَهَ إِلَّا اللَّهُ وَاللَّهُ أَكْبَرُ

"Allah is Pure and all Praise is to Him. Besides Him there is no one worthy of worship, He is the Greatest." (Mishkat, Chapter on Masjid)

When coming out of the Masjid recite Durood and Salaam first and thereafter recite:

رَبِّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ فَضْلِكَ

"O Lord, pardon my sins and open the doors of virtue for me". (Mishkat)

Alternatively recite

اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ

"O Allah, I ask of you, Your Fadl (goodness)."

Also can recite:

In the name of Allah, and prayers and peace be upon the Messenger of Allah. O Allah, I ask You from Your favor. O Allah, guard me from the accursed devil.
[Abu Dawood, Ibn As-Sunnee, Muslim & ibn Maajah]