What dua should I say when entering the mosque?

Enter with the right foot and say: allahumma iftaḥ lī abwāba raḥmatik (O Allah, open for me the doors of Your mercy). When leaving, use the left foot and say: allahumma innī as'aluka min faḍlik (O Allah, I ask You from Your bounty). Reported in Sahih Muslim 713.

Detail

Add the salawat on the Prophet as you enter, following the Sunnah recorded by Ibn al-Sunni.

Pray two rak`as of tahiyyat al-masjid before sitting, if there is time before the iqamah (Sahih al-Bukhari 444).

Leaving with the left foot mirrors the entry rule: right for the noble place, left when leaving.

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