CHAPTER: WHAT IS TO BE RECITED WHILE ENTERING THE MOSQUE
Abu Usaid reported that the Messenger of Allaah (sallAllaahu alayhi wa sallam) said: When any one of you enters the mosque, he should say:" O Allaah! open for me the doors of Thy mercy" ; and when he steps out he should say: 'O Allaah! I beg of Thee Thy Grace." (Imam Muslim said: I heard Yahya saying: I transcribed this hadith from the compilation of Sulaiman b. Bilal.)
A hadith like this has been narrated from the Prophet of Allaah (sallAllaahu alayhi wa sallam) by Abu Usaid.