Islam is a religion of purity and chastity. Its purity is not limited to just the dress and attire. Its purity is not limited to the parts of the body. Its cleanliness is not bound to the home and hearth. These all are but external purity.
Islam insists on purity of the inward aspect (Batin) much more than it insists on purity of the outward aspect (Dhahir). It teaches one to have pure character and attributes. It teaches us to clean our thoughts and notions. It prohibits the filth of ignobleness and teaches us to stay away from evil and unchaste thoughts. It teaches us to stay away indecency and immodesty. Thus, Islam wants us to avoid all kinds of impurity, be it external, internal or of any other kind or type. Almighty Allah says in the Holy Quran:
Say:- My Lord has declared unlawful only (all) the indecent acts which become known. (Surah Al A’raaf: 07:33)
Abstaining from lewd gazing and sweetness of worship
Whenever anyone’s gaze falls on any stranger (non-Mahram) and he/she turns it away, then from the court of Almighty Allah, that person is granted sweetness in worship. As per this Hadith of the the Holy Prophet (Sallallahu alaihi wa sallam)
Translation: It has been narrated on the authority of Hadhrat Abu Umama (May Almighty Allah be well pleased with him) that the Holy Prophet (Sallallahu alaihi wa sallam) said: Whichever Muslim unintentionally sees the beauty of an unrelated (non-Mahram) woman and immediately turns his gaze down, then Almighty Allah will grant him the guidance to perform such worship, the sweetness of which he will actually feel. (Musnad Imam Ahmad, Musnad Abi Umamatal Bahili, Hadith No. 22938)
One of the reasons that the Muslims have been ordered to keep their gaze down is that they should not see any woman. When someone ends up seeing a woman who is without Hijab, then in this situation, it becomes difficult to control one’s base desires (Nafs). That’s why the Holy Prophet (Sallallahu alaihi wa sallam) has ordered the removal of the root cause of all this corruption.
Translation: Surely women come in the guise of Satan and go in the guise of Satan, (i.e. men are easily misguided by women). Hence, whenever anyone from among you sees a woman, who appeals to you, he should come to his wife because this act will remove those thoughts which cross his heart and mind. (Musnad Imam Ahmed, Musnad Jabir, Hadith No. 14911)
Lewd gazing is the prime reason which creates immodesty and is a big reason for indecency. The Lord Almighty Allah has posed restrictions on the very source of indecency.