If some hadiths are combined, the Dua for entering the mosque will be as follows:
أَعُوْذُ بِاللَّهِ الْعَظِيمِ وَبِوَجْهِهِ الْكَرِيمِ وَسُلْطَانِهِ الْقَدِيْمِ مِنَ الشَّيْطَانِ الرَّجِيْمِ، بِسْمِ اللَّهِ وَالصَّلَاةُ وَالسَّلَامُ عَلَى رَسُوْلِ اللَّهِ اَللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ
Pronunciation
A'oodhu billahil-Aatheem wa bi wajhihil-kareem wa sultanihil-qadeem minash-shaytanir-rajeem, bismil-lahi, wassalatu wassalamu 'Aala rasoolil-lah, Allaahumma iftah lee abwaaba rahmatik
Meaning
I take refuge with Allah, The Supreme and with His Noble Face, and His eternal authority from the accursed devil. In the name of Allah, and prayers and peace be upon the Messenger of Allah (ﷺ). O Allah, open the gates of Your mercy for me.
Source: Sahih (Albani). Abu Dawud: 465, 466
Read the hadith
حَدَّثَنَا مُحَمَّدُ بْنُ عُثْمَانَ الدِّمَشْقِيُّ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، - يَعْنِي الدَّرَاوَرْدِيَّ - عَنْ رَبِيعَةَ بْنِ أَبِي عَبْدِ الرَّحْمَنِ، عَنْ عَبْدِ الْمَلِكِ بْنِ سَعِيدِ بْنِ سُوَيْدٍ، قَالَ سَمِعْتُ أَبَا حُمَيْدٍ، أَوْ أَبَا أُسَيْدٍ الأَنْصَارِيَّ يَقُولُ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا دَخَلَ أَحَدُكُمُ الْمَسْجِدَ فَلْيُسَلِّمْ عَلَى النَّبِيِّ صلى الله عليه وسلم ثُمَّ لْيَقُلِ اللَّهُمَّ افْتَحْ لِي أَبْوَابَ رَحْمَتِكَ فَإِذَا خَرَجَ فَلْيَقُلِ اللَّهُمَّ إِنِّي أَسْأَلُكَ مِنْ فَضْلِكَ " .
Abu Usaid al-Ansari reported the Messenger of Allah (May peace be upon him) as saying: when any of you enters the mosque he should invoke blessing on the prophet (ﷺ) and then he should say: O Allah, open to me the gates of thy mercy. And when he goes out, he should say: O Allah, I ask thee out of Thine abundance. [Sahih (Albani). Abu Dawud: 465, 466]
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset