It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Beware of being alone with (unrelated) women. By the One in whose hand is my soul, whenever a man is alone with a woman, Satan comes between them. And for a man to jostle against a pig smeared with mud or filth is better for him than to have his shoulder press against the shoulder of a woman who is not lawful for him."
Hadith Referenceالمعجم الكبير للطبراني / باب الصاد / 7799