(36) باب الْقَوْلِ عِنْدَ دُخُولِ الْمَسْجِدِ وَعِنْدَ الْخُرُوجِ مِنْهُ
It was narrated that 'Abdul-Malik bin Sa'eed said:
"I heard Abu Humaid and Abu Usaid say: 'The Messenger of Allah (ﷺ) 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).'"
Reference: Sunan an-Nasa'i 729
In-book reference: Book 8, Hadith 42
English translation: Vol. 1, Book 8, Hadith 730