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,
وَجَاءَتْ كُلُّ نَفْسٍ مَّعَهَا سَائِقٌ وَشَهِيدٌ (And everybody will come, along with one [ angel ] to drive [ him to the field of reckoning ] and one [ angel ] to testify [ about his deeds ] 50:21). The verse before this depicts the way the Day of Judgment will be established. This verse describes the way in which all human beings will be brought to the plane of Hashr. With every man there will be a Sa'iq and a Shahid. Sa'iq, literally, denotes a person who remains behind a herd of animals or behind a group of people and drives them to a particular place. And Shahid refers to a witness. As for Sa'iq, by the consensus of traditions, it refers to an angel. But there are different views of scholars of Tafsir regarding Shahid. Some say that it too refers to an angel. In this way, there are two angels - Sa'iq and Shahid. Sa'iq's duty is to drive the people to the gathering place, and Shahid's task is to bear witness when the people's deeds will be presented. Another possible interpretation is that these two angels refer to the "honorable scribes" who used to accompany human beings all the time in the world on the right and left to record their deeds. A third possibility is that they refer to some other angels besides the ones mentioned here.
Some scholars interpret Shahid as referring to man's action, and other scholars think that the reference is to man himself. Ibn Kathir opines that the apparent context of the verse indicates that Shahid is also an angel who will bear witness to man's actions. Sayyidna ` Uthman Ibn ` Affan ؓ whilst delivering a sermon recited this verse and said: "Sa` iq will drive every person to Allah, and Shahid will testify about what one has done." Interpreters like Sayyidna Mujahid, Qatadah and Ibn Zaid ؓ placed the same interpretation on the two names. Ibn Jarir has preferred this interpretation.
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.