The Sunnah when entering the mosque is to put the right foot inside the mosque first¹, then say this supplication:

[ اَعُوْذُ بِاللّٰهِ الْعَظِيْمِ وَبِوَجْهِهِ الْكَرِيْمِ وَسُلْطَانِهِ الْقَدِيْمِ مِنَ الشَّيْطَانِ الرَّجِيْمِ ]² [بِسْمِ اللّٰهِ]³ [ وَالصَّلٰوةُ ]⁴ [وَالسَّلَامُ عَلٰى رَسُوْلِ اللّٰهِ]⁵ [ اَللّٰهُمَّ افْتَحْ لِيْ اَبْوَابَ رَحْمَتِكَ ]⁶

I seek refuge in Allah the Magnificent, in His noble Face and in His eternal authority, from Satan the accursed. In the name of Allah (I enter), and blessings and peace be upon Allah's Messenger ﷺ. O Allah! Open for me the doors of Your mercy.

1
Hasan, al-Mustadrak of al-Hakim, Indian edition (1/218), and its chain is hasan, and al-Albani graded it Hasan in “as-Sahihah” no. (2478).
2
Sahih, Sunan Abi Dawud, no. (466), and al-Albani graded it Sahih in “Sahih Abi Dawud” (2/364) no. (485).
3
Da'eef, Sunan Ibn Majah, no. (771), Fadl as-Salat ala an-Nabi ﷺ, no. (82), 'Amal al-Yawm wa al-Laylah of Ibn as-Sunni, no. (88), and al-Albani withdrew his authentication of this wording in “ad-Da'ifah” no. (6953).
4
Da'eef, Sunan at-Tirmidhi, no. (314), 'Amal al-Yawm wa al-Laylah of Ibn as-Sunni, no. (88), Fadl as-Salat ala an-Nabi, no. (82), and al-Albani graded it Hasan in “Takhrij al-Kalim at-Tayyib” no. (64).
5
Sahih, Sunan Abi Dawud, no. (465), Sunan Ibn Majah, no. (772), and al-Albani graded it Sahih in “Sahih Abi Dawud” (2/361) no. (848).
6
Sahih Muslim, no. (713), Sunan Abi Dawud, no. (465), Sunan an-Nasa'i, no. (729), Sunan Ibn Majah, no. (772), the wording is theirs.