Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "The son of Adam's share of zina has been written for him, and he will inevitably meet it. The eyes: their zina is looking; the ears: their zina is listening; the tongue: its zina is speech; the hand: its zina is seizing; the foot: its zina is the steps it takes; and the heart yearns and wishes, and the private part confirms that or denies it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13510