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
It is obligatory upon every Muslim that when entering the mosque, he should send salutations upon you in this manner:
بِسْمِ اللَّهِ وَالسَّلَامُ عَلَىٰ رَسُولِ اللَّهِ اللَّهُمَّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
“In the name of Allah, peace be upon the Messenger of Allah. O Allah! Forgive my sins and open the doors of Your mercy.”
And when leaving the mosque, he should say:
بِسْمِ اللَّهِ وَالسَّلَامُ عَلَىٰ رَسُولِ اللَّهِ اللَّهُمَّ اغْفِرْ لِي ذُنُوبِي وَافْتَحْ لِي أَبْوَابَ فَضْلِكَ
"In the name of Allah. Peace be upon the Messenger of Allah. O Allah! Forgive my sins and open the doors of Your grace for me."
This is a greeting that frees a person from the need to greet near the honored grave. This is a special attribute of the Messenger of Allah in which there is no fear or harm. This is an act that is repeated frequently in prayers. It is also renewed at the time of the call to prayer (Adhan), and every Muslim seeks intercession for the honored Messenger. It is narrated in Sahih Muslim from Abdullah bin Amr bin Al-As (may Allah be pleased with them) that the Noble Prophet (peace be upon him) said:
إذا سمعتم المؤذن فقولوا مثل ما يقول ثم صلوا على فإنه من صلى على مرة صلى الله عليه عشرا ثم سلوا الله لى الوسيلة فإنها درجة فى الجنة لا تنبغى إلا لعبد من عباد الله وأرجو أن أكون أنا هو من سأل لي الوسيلة حلت عليه شفاعتي يوم القيامة
"When you hear the Adhan, say what the Mu'adhdhin says. Then send blessings upon me, for whoever sends blessings upon me once, Allah sends mercy upon him ten times. Then pray for my intercession for me. Intercession is a rank among the degrees of Paradise that is exclusive to only one of Allah's servants, and I hope that I am that servant. Whoever prays for my intercession, his intercession will be permissible for me on the Day of Resurrection."
Reference: (Sahih Muslim - Book of Prayer: Chapter on the recommendation to say what the Mu'adhdhin says, Hadith: 384)
