Allah encompasses and watches all of Man's Activity
Allah the Exalted affirms His absolute dominance over mankind, being their Creator and the Knower of everything about them. Allah the Exalted has complete knowledge of all thoughts that cross the mind of man, be they good or evil. In the Sahih, the Messenger of Allah ﷺ said,
(Verily, Allah the Exalted has forgiven my Ummah (Muslims) for what they talk (think) to themselves about, as long as they do not utter or implement it.) The statement of Allah the Exalted,
مَّا يَلْفِظُ مِن قَوْلٍ إِلَّا لَدَيْهِ رَقِيبٌ عَتِيدٌ (Not a single word is uttered by one, but there is a watcher near him, ready [ to record ]...50:18) In other words, there is always a supervising angel [ the scribe ] whose responsibility is to record any word that the human utters or speaks. Sayyidna Hasan Basri and Qatadah رحمۃ اللہ علیہما said that this angel records every single word - whether or not it is sinful or reward-able. Ibn ` Abbas ؓ said that only those utterances are recorded that are sinful or rewardable. Ibn Kathir, having quoted both these views, preferred the first view on the basis of the general words of the verse that every utterance is recorded. According to another narration of Sayyidna Abu Talhah ؓ from Ibn ` Abbas ؓ both the views can be accommodated. This narration states that in the first instance every utterance is recorded, whether or not it is sinful or rewardable. Once a week on Thursdays, the angels review the recorded utterances and retain only those that are sinful or rewardable, good or bad; the rest are discounted. Thus the Qur'an says: يَمْحُو اللَّـهُ مَا يَشَاءُ وَيُثْبِتُ "Allah wipes off what He wills and affirms (what He wills). And with Him is the Mother Book." (Ar-Ra` d 39)
Imam Ahmad (رح) has reported from Sayyidna Bilal Ibn Harith Al-Muzani ؓ that the Messenger of Allah ﷺ has said:
"Verily, a man sometime utters a good word that pleases Allah, unaware of how highly it wi11 be rewarded, and on its account Allah decrees His pleasure of him until the Day he meets Him. Similarly, a man might utter a word that angers Allah, unaware of how dreadful its punishment will be, and on its account Allah decrees for him His anger until the Day he meets Him."
Sayyidna ` Alqamah ؓ ، after narrating this from Bilal Ibn Harith ؓ ، used to say: "How many words did I not utter because of this hadith." (Ibn Kathir)
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 18–28
The study of this world shows that there is an unerring system of ‘recording’ in operation here. Man’s thoughts are impressed on the membrane of his brain. Every utterance a man makes is permanently preserved in the shape of sound waves in the air. Man’s actions are preserved in the external world by means of heat waves in such a way that they can be repeated at any time. All these are known facts of today, and these known facts are what make the Quran’s claim credible that man’s intentions, his utterances and his actions, are all in the knowledge of his Creator. All affairs of all human beings are entered in the registers of the angels.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.