"The Messenger of Allah said: 'When anyone of you enters the mosque, let him send peace upon the prophet, then let him say: "Allahummaftah li abwaba rahmatika (O Allah, open to me the gates of Your mercy)." And when he leaves, let him say: "Allahumma inni as'aluka min fadlika. (O Allah, I ask of you from Your bounty)." (Using translation from Ibn Mājah 772)  

قَالَ رَسُولُ اللهِ ﷺ إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيَقُلْ اللهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ وَإِذَا خَرَجَ فَلْيَقُلْ اللهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ