Umar reported: The Messenger of Allah, peace and blessings be upon him, said, “I enjoin you to follow my companions, then those after them and those after them. Thereafter, falsehood will spread until a man offers his oath without being asked and a witness gives testimony without being asked. No doubt, a man is not in seclusion with a woman but that Satan is the third of them. Hold fast to the community and beware of division, for Satan is with one alone and he is further from two together. Whoever wants the best place in Paradise, let him hold fast to the community. Whoever rejoices with his good deeds and is saddened by his evil deeds is the believer among you.”
Source: Sunan al-Tirmidhī 2165
Grade: Sahih (authentic) according to Al-Albani
عن عمر قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أُوصِيكُمْ بِأَصْحَابِي ثُمَّ الَّذِينَ يَلُونَهُمْ ثُمَّ الَّذِينَ يَلُونَهُمْ ثُمَّ يَفْشُو الْكَذِبُ حَتَّى يَحْلِفَ الرَّجُلُ وَلَا يُسْتَحْلَفُ وَيَشْهَدَ الشَّاهِدُ وَلَا يُسْتَشْهَدُ أَلَا لَا يَخْلُوَنَّ رَجُلٌ بِامْرَأَةٍ إِلَّا كَانَ ثَالِثَهُمَا الشَّيْطَانُ عَلَيْكُمْ بِالْجَمَاعَةِ وَإِيَّاكُمْ وَالْفُرْقَةَ فَإِنَّ الشَّيْطَانَ مَعَ الْوَاحِدِ وَهُوَ مِنْ الِاثْنَيْنِ أَبْعَدُ مَنْ أَرَادَ بُحْبُوحَةَ الْجَنَّةِ فَلْيَلْزَمْ الْجَمَاعَةَ مَنْ سَرَّتْهُ حَسَنَتُهُ وَسَاءَتْهُ سَيِّئَتُهُ فَذَلِكُمْ الْمُؤْمِنُ
2165 سنن الترمذي كتاب الفتن باب ما جاء في لزوم الجماعة
2165 المحدث الألباني خلاصة حكم المحدث صحيح في صحيح الترمذي