Ibn Abbas (may Allah be pleased with them both) said: I have not seen anything more resembling minor sins (al-lamam) than what Abu Hurayrah (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) said: "Allah has written for the son of Adam his share of zina (fornication), which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, the zina of the soul is wishing and desiring, and the private part confirms that or denies it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13509