Hadith 10:39
‘Umar reported the Prophet as saying, "Whenever a man is alone with a woman the devil makes a third.” Tirmidhi transmitted it.
وَعَنْ عُمَرَ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَا يَخْلُوَنَّ رَجُلٌ بِامْرَأَةٍ إِلَّا كَانَ ثالثهما الشَّيْطَان» . رَوَاهُ التِّرْمِذِيّ
Original Reference: Mishkat al-Masabih 3118