Hadith: When the Prophet (may Allah's peace and blessings be upon him) entered the mosque, he would say: "a‘ūdhu billāhi al-‘azhīm wa biwajhihi al-karīm wa sultānihi al-qadīm min ash-shaytān ar-rajīm" (I seek refuge in Allah, the Majestic, and in His noble Countenance, and in His eternal dominion, from the accursed devil)

عن عبد الله بن عمرو بن العاص رضي الله عنهما:
عن النبي صلى الله عليه وسلم أنه كان إذا دخل المسجد قال: «أعوذ بالله العظيم، وبوجهه الكريم، وسلطانه القديم، من الشيطان الرَّجِيم»، قال: أَقَطُّ؟ قلت: نعم، قال: فإذا قال ذلك قال الشيطان: حُفِظَ منِّي سائر اليوم.
[حسن] - [رواه أبو داود] - [سنن أبي داود: 466]

‘Abdullāh ibn ‘Amr ibn al-‘Ās (may Allah be pleased with him) reported:
When the Prophet (may Allah's peace and blessings be upon him) entered the mosque, he would say: "a‘ūdhu billāhi al-‘azhīm wa biwajhihi al-karīm wa sultānihi al-qadīm min ash-shaytān ar-rajīm" (I seek refuge in Allah, the Majestic, and in His noble Countenance, and in His eternal dominion, from the accursed devil) He said: "Is that all?" I said: 'Yes.' He said: "And when he says that, the devil says: "He is protected from me for the rest of the day."
[Good hadith] - [Narrated by Abu Daoud] - [Sunan Abu Daoud - 466]

Explanation

When the Prophet (may Allah's peace and blessings be upon him) entered the mosque, he would say: "I seek refuge with Allah, the Majestic" i.e., I resort and turn for help to Allah and His attributes. "and in His noble Countenance" i.e., the Generous Giver. "and in His eternal dominion" i.e., His dominance, power, and the ability to vanquish anyone of His creation He wills. 'eternal' i.e., permanent, timeless. "from the accursed devil", who is banished and expelled from the mercy of Allah. i.e., O Allah, protect me from his whisperings, his temptations, his steps, his ideas, his insinuations, and his misguidance, for he is the one who causes misguidance and leads to temptation and ignorance. It was said to ‘Abdullāh ibn ‘Amr: "Is that all?" i.e., did the Prophet (may Allah's peace and blessings be upon him) say that only? He said: Yes.
If a person says this supplication upon entering the mosque, the devil will say: He has protected himself from me the whole time, during the day and night.