The Sunnah when leaving the mosque is to put the left foot outside the mosque first¹, then say this supplication:

[بِسْمِ اللّٰهِ]² [وَالصَّلٰوةُ ]³ [وَالسَّلَامُ عَلٰى رَسُوْلِ اللّٰهِ]⁴ [اَللّٰهُمَّ اِنِّيْٓ اَسْاَلُكَ مِنْ فَضْلِكَ]⁵، [اَللّٰهُمَّ اعْصِمْنِيْ مِنَ الشَّيْطَانِ الرَّجِيْمِ ]⁶

In the name of Allah (I leave), and blessings and peace be upon Allah's Messenger ﷺ. O Allah! I ask You for Your bounty. O Allah! Protect me from Satan the accursed.

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
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).
3
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).
4
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).
5
Sahih Muslim, no. (713), Sunan Abi Dawud, no. (465), Sunan an-Nasa'i, no. (729), Sunan Ibn Majah, no. (772), the wording is theirs.
6
Maqtu', Sunan Ibn Majah no. (773). This is neither a hadith of the Prophet ﷺ nor the saying of a Companion; rather, according to the research of Imam an-Nasa'i (RH), it is the saying of Ka'b al-Ahbar [Amal al-Yawm wa al-Laylah of an-Nasa'i (page 179)], and that is correct. Ibn Hajar (RH) also supported Imam an-Nasa'i. [Nata'ij al-Afkar of Ibn Hajar: 1/277]
This is also the finding of Allamah Muqbil (RH); see: [Ahadith Mu'allah Zahiruha as-Sihhah (page 434)]
Allamah al-Albani (RH) did not discuss this defect, which shows that he was not aware of it, and that is why he graded it Sahih as marfu'. Allah knows best.