"Come not near to unlawful sex. Verily, it is Fahishah (immoral sin) and an evil way." - Surah No.17 Al-Isra , Aaayat 32
Allah says, forbidding His servants to commit Zina or to approach it or to do anything that may lead to it:
(And come not near to unlawful sex. Verily, it is a Fahishah (immoral sin)) meaning a major sin,
(and an evil way.) meaning, a terrible way to behave.
Imam Ahmad recorded Abu Umamah saying that a young man came to the Prophet and said,
"O Messenger of Allah! Give me permission to commit Zina (unlawful sex).'' The people surrounded him and rebuked him, saying, "Stop! Stop!'' But the Prophet said,
(Come close) The young man came to him, and he said,
(Sit down) so he sat down. The Prophet said,
(Would you like it (unlawful sex) for your mother) He said, "No, by Allah, may I be ransomed for you.'' The Prophet said,
(Neither do the people like it for their mothers.) The Prophet said,
(Would you like it for your daughter) He said, "No, by Allah, may I be ransomed for you.'' The Prophet said,
(Neither do the people like it for their daughters. ) The Prophet said,
(Would you like it for your sister) He said, "No, by Allah, may I be ransomed for you.'' The Prophet said,
(Neither do the people like it for their sisters.) The Prophet said,
(Would you like it for your paternal aunt) He said, "No, by Allah, O Allah's Messenger! may I be ransomed for you.'' The Prophet said,
(Neither do the people like it for their paternal aunts.) The Prophet said,
(Would you like it for your maternal aunt) He said, "No, by Allah, O Allah's Messenger! may I be ransomed for you.'' The Prophet said,
(Neither do the people like it for their maternal aunts.) Then the Prophet put his hand on him and said,
(O Allah, forgive his sin, purify his heart and guard his chastity.) After that the young man never paid attention to anything of that nature.
Source: Tafseer Ibn Katheer (Surah No.17 Al-Isra , Aaayat 32), Darussalam English Publication
