Is it a sin to have sex (man) before marriage according to the Holy Quran and the Islamic rules of religion?

What is considered Zina?

Praise be to Allaah. 

Zina (adultery, fornication) does not refer only to penetration, rather there is the zina of the hand, which is touching that which is forbidden, and the zina of the eyes, which is looking at that which is forbidden, even though zina that is committed with the private parts, is the zina which is punishable with the hadd punishment.

It was narrated from Abu Hurayrah that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah has decreed for every son of Adam his share of zina, which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, one may wish and desire, and the private parts confirm that or deny it.”

[Bukhari & Muslim]

"And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way."

Surat Al-'Isra' [17:32] - The Noble Qur'an - القرآن الكريم

“And those who invoke not any other ilaah (god) along with Allaah, nor kill such person as Allaah has forbidden, except for just cause, nor commit illegal sexual intercourse ___ and whoever does this shall receive the punishment.

The torment will be doubled to him on the Day of Resurrection, and he will abide therein in disgrace;

Except those who repent and believe (in Islamic Monotheism), and do righteous deeds; for those, Allaah will change their sins into good deeds, and Allaah is Oft‑Forgiving, Most Merciful”

Surat Al-Furqan [25:68-70] - The Noble Qur'an - القرآن الكريم

“And verily, I am indeed forgiving to him who repents, believes (in My Oneness, and associates none in worship with Me) and does righteous good deeds, and then remains constant in doing them (till his death)”

Surat Taha [20:82] - The Noble Qur'an - القرآن الكريم

“Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts). That is purer for them. Verily, Allaah is All‑Aware of what they do.

And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts)”

Surat An-Nur [24:30-31] - The Noble Qur'an - القرآن الكريم

References:
- The Qur'an
- Ruling on the things that lead to zina (IslamQA)
- They committed zina then they got married, and they are asking whether the marriage is valid (IslamQA)