I understand very well what is zina of the eyes and hands. It is seeing and touching Haram things. But what is zina of the heart? Is it confined to imagining Haram things like: Just like it is Haram to look at the arms of non mahram women, it is equally Haram to imagine her arms in your imagination? In other words all those things that are Haram to see with the eyes of your face, are equally Haram to see with the eyes of your imagination. Please explain all the things that are included in the zina of the heart. So I won't miss one of them and commit zina of the heart without understanding it.
What you have stated about the Zina of the heart is not totally true since it is not only about what comes in your mind but what your heart also feels for such imaginations.
Its definition: what one's wants and desires of Haram things without actually committing them, like being inclined toward Zina (adultery) and becoming infatuated with women.
It has been mentioned in the Hadith where it was narrated from Abu Hurayrah that the Prophet (Peace Be Upon Him) said:
It has been written upon the Son of Adam (the Muslim) his share of adultery which he will indulge in; there is no escaping it. The two eyes, their adultery is looking (at what is forbidden). The two ears, their adultery is listening. The tongue, its adultery is speaking. The hand, its adultery is the touching, and the foot, its adultery is walking (toward adultery). The heart desires and hopes (for adultery) but the private parts either confirm that or deny it.
Here is the same Hadith in Arabic (which was the language of the Prophet (Peace Be Upon Him) and his people at that time).
Also, it is stated in Surah Isra (17) — Verse 36:
In Arabic (the exact wording):
إِنَّ السَّمْعَ وَالْبَصَرَ وَالْفُؤَادَ كُلُّ أُوْلَئِكَ كَانَ عَنْهُ مَسْؤُولاً...
In English (in what it means):
...Indeed, the hearing, the sight and the heart - about all those [one] will be questioned.
The verse indicates clearly that you are responsible for your heart, and you will be asked about in the Day of Judgement.
The Zina of the heart is Haram in Islam even though it doesn't have a specific punishment that is under Islamic law (Shariah) which are mandated and fixed by Allah, and are known as Hadd (singular) - Hudud (plural).
Again, the meaning of the Zina of the heart is what comes to your mind of imagining Haram things and doings, such as non-Mahrams figures with feeling attached to that imagination, is considered of the heart Zina.
Zina and zina of the heart
Allah the Almighty says:
And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way. (17:32)
This simply means that we avoid anything that would get us close to zina, be it by avoiding places where there's a mixing of genders, be it by simply avoiding a too close look on a female/male body without necessity etc. or more general to avoid any temptation that might get us close to zina.
In the sunnah we find:
"Allah has written for the son of Adam his inevitable share of adultery whether he is aware of it or not: The adultery of the eye is the looking (at something which is sinful to look at), and the adultery of the tongue is to utter (what it is unlawful to utter), and the innerself wishes and longs for (adultery) and the private parts turn that into reality or refrain from submitting to the temptation." (Sahih al-Bukhari)
While imam Muslim compiled a similar narration saying:
Allah fixed the very portion of adultery which a man will indulge in. There would be no escape from it. The adultery of the eye is the lustful look and the adultery of the ears is listening to voluptuous (song or talk) and the adultery of the tongue is licentious speech and the adultery of the hand is the lustful grip (embrace) and the adultery of the feet is to walk (to the place) where he intends to commit adultery and the heart yearns and desires which he may or may not put into effect. (Sahih Muslim)
Simply speaking the ahadith above mean: that each human being will inevitably commit at least a part of adultery (as Allah prescribed it for him) among us there will be people who will commit zina in real by having intercourse (with penetration), while there will be others who will commit it by allegory for example by unlawful looks (looking lustfully or more than necessary at a foreign person of the opposite gender -or a person that may cause a sexual temptation in general-), or by listening to zina or whatever is related to realize it (for example hearing a story of somebody who performed it), or by touching (a foreign person of the opposite gender) with his hand, or by going to a place (where people commit zina or to perform zina), or by kissing a foreign person (of the opposite gender), or by unlawful chat with a foreign person (of the opposite gender), or by unlawful thoughts (in the heart: in the inside) and all this may either lead him to commit zina (physically by having intercourse) or he might come to mind repent and leave it. Therefore the prophet () referred to these "actions" of the body parts as "zina" as they are introducing it or leading to it.
For details you may refer to the hadith commentaries of imam an-Nawawi on Sahih Muslim (Al-Minhaj see -here in Arabic-), of Mulla 'Ali al-Qari called Mirqaat al-Mafateeh (See here in Arabic) and in Fath al-Bari (see here in Arabic) of ibn Hajar al-'Asqalani.
Basically the zina of the heart as referred to in the version narrated by imam Muslim (and in similar words by imam Ahmad and abu Dawod) is that of the innerself (nafs) which is referred to in other narrations like that which al-Bukhari compiled twice in his Sahih and which was also compiled by Muslim and abu Dawod.
The zina or crime that our hearts or innerself commit is that of wishing and longing and thinking (Imagination) lustfully of a foreign person, in a manner that we actually are tempted to commit "real" zina.
Be aware that if wife or husband thought of each other lustfully this certainly is not zina of the heart.
Note that al-Qari and other scholars said that what constitutes zina here is not the simple wishing, but a persistence whishing as else we would a conflict with the sahih hadith saying:
"Allah forgives my followers those (evil deeds) their souls may whisper or suggest to them as long as they do not act (on it) or speak." (See for example in Sahih al-Bukhari)
Is there a punishment?
As for punishments or hudud (plural of had) in Islam they are restricted and applied only for those misdeeds or crimes that go against the five fundamental goals of shari'a:
The goal of shari'a is the preservation of ...
- Religion / Faith (din الدين) as a crime against it could we could consider the case of an apostate that attacks Islam and Muslims.
- Life (nafs النفس) a crime against it is the case of a person who murders innocent people.
- Lineage / Progeny (nasl النسل) a crime against it is zina, as it may create children who have no father or who are named after the wrong father and it may create incest issues (father unknown).
- Intellect (‘aql العقل) an example for a crime against it is drinking/selling alcohol
- Property / Wealth (mal المال) the example for that is stealing.
Zina of the heart doesn't fall in any of these five categories therefore it is not a punishable crime. But it is a misdeed one must repent from.
Zina of eyes, hands isn't like real zina (sexual intercourse).
Because looking with eyes and touching may lead you to real zina that is why it's prohhibited but if you did any of these zina unavoidably, unintentionally that is forgiven and that definitely will not take you towards real zina if your intention was not.