Have they not looked at the birds above them, with wings outspread and folded back? Nothing holds them up but the All-Merciful. He is the All-Seeing. (Surat al-Mulk, 67:19)
Our human faculty of sight is limited. With the naked eye, we can see for only a few kilometers. But to achieve this, we need clear weather and to be standing on a high place. Yet no matter how convenient the conditions may be, we can see only a hazy image.
In some situations, and especially when people are alone, they assume that nobody sees them. Assuming that they will not have to account for their deeds, they feel encouraged to do whatever they wish. Furthermore, they think they will never suffer any consequence for their wrongdoing. But this is a great delusion, for Allah sees even the smallest act. While our eyesight is limited, Allah sees the room in which a person remains, the rooms surrounding that room, the entire house, the city and country that harbor that house, the continent in which that country is located, the entire Earth, all planets, space, and all other dimensions beyond it. In the Qur'an, Allah informs us that He is aware of everything:
You do not engage in any matter, recite any of the Qur'an, or do any action without Our witnessing you while you are occupied with it. Not even the smallest speck eludes your Lord, either on Earth or in Heaven. Nor is there anything smaller than that, or larger, which is not in a Clear Book. (Surah Yunus, 10:61)
Perform prayer and pay alms. Any good you send ahead for yourselves, you will find with Allah. Certainly Allah sees what you do. (Surat al-Baqara, 2:110)
They have different ranks with Allah. Allah sees what they do. (Surah Al ‘Imran, 3:163)
Those who adulterate Our Signs are not concealed from Us. Who is better—someone who will be thrown into the Fire or someone who will arrive in safety on the Day of Rising? Do what you like. He sees whatever you do. (Surah Fussilat, 41:40)