أَخْبَرَنَا سُلَيْمَانُ بْنُ عُبَيْدِ اللَّهِ الْغَيْلاَنِيُّ، - بَصْرِيٌّ - قَالَ حَدَّثَنَا أَبُو عَامِرٍ، قَالَ حَدَّثَنَا سُلَيْمَانُ، عَنْ رَبِيعَةَ، عَنْ عَبْدِ الْمَلِكِ بْنِ سَعِيدٍ، قَالَ سَمِعْتُ أَبَا حُمَيْدٍ، وَأَبَا، أُسَيْدٍ يَقُولاَنِ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيَقُلِ اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ وَإِذَا خَرَجَ فَلْيَقُلِ اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ " .
It was narrated that 'Abdul-Malik bin Sa'eed said: "I heard Abu Humaid and Abu Usaid say: 'The Messenger of Allah (PBUH) said: "When any one of you enters the Masjid, let him say: 'Allahumma aftahli abwaba rahmatik (O Allah, open to me the gates of your mercy). And when he leaves let him say: Allahumma inni as'aluka min fadlik (O Allah, I ask You of Your bounty).'" Translation Not Available
Book Ref: Sunan An Nisai Book 8 Hadith 729
Web Ref: Sunan An Nisai Vol 1 Book 8 Hadith 730