Dua When Entering the Mosque
اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
Meaning: O Allah, open for me the doors of Your mercy.
Sahih Muslim 713
The specific supplications the Prophet taught for entering and leaving the mosque. Right foot first when entering, left foot first when leaving.
اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ
The Prophet said: "Whoever goes to the mosque in the morning and evening, Allah prepares for him a place in Paradise for every morning and evening." (Bukhari 662)