This excerpt is taken from Sheikh al-Islam Imam Ibn Taymiyyah's book Al-Jawab al-Bahir fi Zuwar al-Maqabir, translated by Sheikh Ataullah Saqib.
The method of sending salutations upon the Messenger of Allah when entering and leaving the mosque
The second type is that when entering and leaving the mosque, you are greeted with salam, as narrated in Musnad and Sunan from Fatimah al-Zahra (may Allah be pleased with her) that the Noble Messenger (peace and blessings of Allah be upon him and his family) said that when any one of you enters the mosque, he should say:
بِسْمِ اللَّهِ وَالسَّلَامُ عَلَىٰ رَسُولِ اللَّهِ اللَّهُمَّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
"In the name of Allah and peace be upon the Messenger of Allah, O Allah! Forgive my sins and open the doors of Your mercy for me."
And when leaving the mosque, he should say this supplication:
بِسْمِ اللَّهِ وَالسَّلَامُ عَلَىٰ رَسُولِ اللَّهِ اللَّهُمَّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ فَضْلِكَ
"In the name of Allah and peace be upon the Messenger of Allah, O Allah! Forgive my sins and open the doors of Your bounty for me."
Reference: (Musnad Ahmad 282/2, Sunan al-Tirmidhi, Book of Prayer: Chapter on what to say when entering the mosque, Hadith: 314, Sunan Ibn Majah, Book of Mosques: Chapter on supplication when entering the mosque, Hadith: 771)
It is narrated in Sahih Muslim that reciting the above supplication when entering the mosque is a confirmed Sunnah. For this reason, scholars have written in their respective books of rituals that it is very necessary for a person entering the Prophet's Mosque to recite the above supplication. Therefore, greeting the Prophet (peace and blessings of Allah be upon him) with salam when entering and leaving the mosque and during prayer is more virtuous than greeting the honored grave with salam. This is entirely beneficial and carries no risk of harm; Allah is pleased with it and grants its reward to the Prophet (peace and blessings of Allah be upon him) and all the believers as well.
