Al-Kāfi - Volume 6 > The Undesirable Case of One’s Sleeping Alone and the Characteristics Prohibited because of its Frightfulness
Hadith #3

مُحَمَّدُ بْنُ يَحْيَى عَنْ عَبْدِ اللَّهِ بْنِ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ الْحَكَمِ عَنْ أَبَانٍ الْأَحْمَرِ عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَبِي جَعْفَرٍ ( عليه السلام ) قَالَ قَالَ إِنَّ الشَّيْطَانَ أَشَدَّ مَا يَهُمُّ بِالْإِنْسَانِ حِينَ يَكُونُ وَحْدَهُ خَالِياً لَا أَرَى أَنْ يَرْقُدَ وَحْدَهُ .

3. Muhammad ibn Yahya has narrated from ‘Abd Allah ibn Muhammad from Ali ibn al-Hakam from Aban al-Ahmar from Muhammad ibn Muslim who has said the following: “Abu Ja‘far, ‘Alayhi al-Salam, has said that Satan is the strongest against man when he is alone without anything and I say that he must not sleep alone.’”