When one of you enters the Masjid, let him say:

TRANSLATION:
O Allah, open to me the gates of Your mercy.

Narrated: Fatimah (ra)
Whenever the Messenger of Allah (ﷺ) entered the mosque he would say:

TRANSLATION:
In the Name of Allah, and peace be upon the Messenger of Allah. O Allah, forgive me my sins and open to me the gates of Your mercy.

Haiwah bin Shuraih said:
"I met 'Uqbah bin Muslim, and told him: 'I have heard that you narrate from 'Abdullah bin 'Amr bin Al- 'As that the Prophet (ﷺ) would say, when he entered the Masjid:

He ('Uqbah) said: 'Is that all?' I said: 'Yes.' So he (added): 'So when he says that, Shaitan says: "He has been protected from me for the entire day."

TRANSLATION:
I seek refuge in Allah, the Great, and in His Noble Face and His Eternal Power, from Shaitan, the rejected.

It was narrated from 'Abdullâh bin 'Abbas that he slept at the house of the Messenger of Allah (ﷺ). He (ﷺ) woke up, cleaned his teeth with the Siwak and performed Wudu' while saying: "(Quran 3;190) Verily, in the creation of the heavens and the earth, and in the alternation of night and day, there are indeed signs for men of understanding.", and he (ﷺ) recited these verses until the end of the Surah. Then he (ﷺ) stood and prayed two Rak'ah, in which he (ﷺ) stood, bowed and prostrated for a long time. Then he (ﷺ) went and slept until he (ﷺ) started to breathe deeply. Then he (ﷺ) did that three times, six Rak'ah in all, cleaning his (ﷺ) teeth with the Siwak, performing Wudu and reciting these verses. Then he (ﷺ) prayed Witr with three Rak'ah. Then the Mu 'adhdhin called the Adhan and he (ﷺ) went out to pray, saying:

TRANSLATION;
O Allah,
1) Put in my heart light and on my tongue light,
2) Put in my hearing light, put in my seeing light,
3) Put behind me light and in front of me light,
4) Put above me light and below me light.
O Allah, give me light.

PROPHET MUHAMMAD (ﷺ) said:
Whoever enters the marketplace and say:

1) Allah shall record a million good deed for him,
2) Wipe a million evil deeds away from him,
3) Raise a million ranks for him and b
4) Build a house in Paradise for him.

TRANSLATION:
There is none worthy of worship except Allah, Alone, without partner, to Him belongs the dominion, and to Him is all the praise, He gives life and causes death, He is Living and does not die, in His Hand is the good, and He has power over all things.

PROPHET MUHAMMAD (ﷺ) said:
When he leaves (masjid), let him say:

TRANSLATION:
O Allah, I ask You of Your Bounty.

Narrated : Fatimah (ra)
When he (ﷺ) left (the masjid) he would say:

TRANSLATION:
In the Name of Allah, and peace be upon the Messenger of Allah. O Allah, forgive me my sins and open to me the gates of Your bounty.

PROPHET MUHAMMAD (ﷺ) said:
When he leaves (the masjid), let him send peace upon the Prophet and say:

TRANSLATION:
O Allah, protect me from the accursed Shaitan.