مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 3118
Marriage
Looking at a Woman who is asked in Marriage, and an Explanation of what may not be seen - Section 2
وَعَنْ عُمَرَ عَنِ النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَا يَخْلُوَنَّ رَجُلٌ بِامْرَأَةٍ إِلَّا كَانَ ثالثهما الشَّيْطَان» . رَوَاهُ التِّرْمِذِيّ
‘Umar reported the Prophet (ﷺ) as saying, "Whenever a man is alone with a woman the devil makes a third.” Tirmidhi transmitted it.
Mishkat al-Masabih, 3118صَحِيح (الألباني)