These are the supplications for entering and leaving the mosque and for responding to the adhan, including the well-known dua after the adhan for the Prophet’s intercession.
اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
Transliteration: Allahummaftah li abwaba rahmatik.
Translation: O Allah, open for me the gates of Your mercy.
Source: Sahih Muslim 713
When to recite: When entering the masjid, stepping in with the right foot; and the second dua when leaving, stepping out with the left foot. This is the Sunnah in a hadith of Anas, graded hasan by al-Albani (as-Silsilah as-Sahihah 2478).
اللَّهُمَّ رَبَّ هَذِهِ الدَّعْوَةِ التَّامَّةِ، وَالصَّلَاةِ الْقَائِمَةِ، آتِ مُحَمَّدًا الْوَسِيلَةَ وَالْفَضِيلَةَ، وَابْعَثْهُ مَقَامًا مَحْمُودًا الَّذِي وَعَدْتَهُ
Transliteration: Allahumma Rabba hadhihid-da'watit-tammah, was-salatil-qa'imah, ati Muhammadanil-wasilata wal-fadilah, wab'athhu maqaman mahmudanil-ladhi wa'adtah.
Translation: O Allah, Lord of this perfect call and of the prayer about to be established, grant Muhammad al-Wasilah (the highest station in Paradise) and excellence, and raise him to the praised station You have promised him.
Source: Sahih al-Bukhari 614
When to recite: After the adhan has finished, having repeated its words after the mu’adhdhin.
أَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ، رَضِيتُ بِاللَّهِ رَبًّا، وَبِمُحَمَّدٍ رَسُولًا، وَبِالْإِسْلَامِ دِينًا
Transliteration: Ash-hadu an la ilaha illallahu wahdahu la sharika lah, wa anna Muhammadan 'abduhu wa rasuluh, raditu billahi Rabba, wa bi Muhammadin rasula, wa bil-Islami dina.
Translation: I bear witness that there is no god but Allah alone, without partner, and that Muhammad is His servant and Messenger. I am pleased with Allah as my Lord, with Muhammad as Messenger and with Islam as my religion.
Source: Sahih Muslim 386
When to recite: On hearing the mu’adhdhin, after he says the two testimonies of faith.
Hadith references follow the standard numbering on sunnah.com. Quranic Arabic is the Uthmani text with the Pickthall (1930, public domain) translation; other translations are our own.