The mosque is the place of worship, the house of Allah (swt). Due to the significance of this Holy house, when a Muslim is entering the Mosque, he has to follow the instructions laid down by the Messenger of Allah.
Some of these laid down procedures include the Supplication to recite before entering the Mosque according to our Beloved Prophet (pbuh).
First of all, while entering the Mosque the Muslim whould enter with his right leg first saying the following Du’a:
Faitma, daughter of the Prophet (pbuh) said, “Whenever Allah’s Messenger entered the mosque, he would utter:
“BISMILLAAHI WAS-SALAAMU ‘ALAA RASOOLILLAAHI. ALLAAHUMMAGH FIRLEE ZUNOOBEE, WAFTAH-LEE ABWAABA RAHMATIKA.”
“In the name of Allah, Peace be upon Allah’s Messenger. Oh Allah! Forgive me my sins and open for me the doors of your Mercy.”
And when he comes out of the Mosque he would pray:
“BISMILLAAHI WAS-SALAAMU ‘ALAA RASOOLIL LAAHI. ALLAAHUMAGH FIRLEE ZUNOOBEE, WAFTAH-LEE ABWAABA FADLIK.”