✅Answer: The matter is mentioned in the following two narrations:

1 عَنْ عَبْدِ اللَّهِ بْنِ بُكَيْرٍ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: كَانَ عَلِيٌّ قَدِ اتَّخَذَ بَيْتاً فِي دَارِهِ لَيْسَ بِالْكَبِيرِ وَ لَا بِالصَّغِيرِ وَ كَانَ إِذَا أَرَادَ أَنْ يُصَلِّيَ مِنْ آخِرِ اللَّيْلِ أَخَذَ مَعَهُ صَبِيّاً لَا يَحْتَشِمُ مِنْهُ ثُمَّ يَذْهَبُ إِلَى ذَلِكَ الْبَيْتِ فَيُصَلِّي.

📚 Qurb al-Isnad, p.75

Imam Sadiq (peace be upon him) said: Imam Ali (peace be upon him) had a room in his house that was neither small nor large. When he wanted to pray at night, he would take a child who would not be embarrassed by him and go to that room and pray!

2 عَنْ عُبَيْدِ بْنِ زُرَارَةَ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: كَانَ عَلِيٌّ قَدْ جَعَلَ بَيْتاً فِي دَارِهِ لَيْسَ بِالصَّغِيرِ وَ لَا بِالْكَبِيرِ لِصَلَاتِهِ وَ كَانَ إِذَا كَانَ اللَّيْلُ ذَهَبَ مَعَهُ بِصَبِيٍّ لِيَبِيتُ مَعَهُ فَيُصَلِّي فِيهِ.

📚 Al-Mahasin, p.612

Imam Sadiq (peace be upon him) said: Imam Ali (peace be upon him) had a room in his house for prayer, neither small nor large. When night came, he would take a child with him to spend the night there (sleep in that room) while he prayed in that room!

✅From the above narrations, it is clear that Imam Ali (peace be upon him) would take a child with him when he wanted to pray at night in the mosque of his house, so that the child would stay there and he could pray. The reason that can be inferred from the narrations is that being alone in a room is discouraged, and Imam Ali (peace be upon him) took the child along to avoid being alone. Consider the following two narrations:

3 عَنْ مُحَمَّدِ بْنِ مُسْلِمٍ عَنْ أَحَدِهِمَا أَنَّهُ قَالَ: لَا تَشْرَبْ وَ أَنْتَ قَائِمٌ وَ لَا تَبُلْ فِي مَاءٍ نَقِيعٍ وَ لَا تَطُفْ بِقَبْرٍ وَ لَا تَخْلُ فِي بَيْتٍ وَحْدَكَ وَ لَا تَمْشِ بِنَعْلٍ وَاحِدَةٍ فَإِنَّ الشَّيْطَانَ أَسْرَعُ مَا يَكُونُ إِلَى الْعَبْدِ إِذَا كَانَ عَلَى بَعْضِ هَذِهِ الْأَحْوَالِ وَ قَالَ إِنَّهُ مَا أَصَابَ أَحَداً شَيْءٌ عَلَى هَذِهِ الْحَالِ فَكَادَ أَنْ يُفَارِقَهُ إِلَّا أَنْ يَشَاءَ اللَّهُ عَزَّ وَ جَلَّ.

📚 Al-Kafi, Vol.6, p.534

Imam Baqir or Imam Sadiq (peace be upon them) said: Do not drink while standing, do not urinate in stagnant water, do not defecate on a grave, do not be alone in a room, and do not walk with one slipper, because Satan’s fastest opportunity to approach a servant is when they are in one of these situations. And he said: Nothing will approach a person in these situations except by the will of Allah Almighty!

4 عَنِ ابْنِ الْقَدَّاحِ عَنْ أَبِيهِ مَيْمُونٍ عَنْ أَبِي جَعْفَرٍ أَنَّهُ قَالَ لِمُحَمَّدِ بْنِ سُلَيْمَانَ أَيْنَ نَزَلْتَ قَالَ فِي مَكَانِ كَذَا وَ كَذَا قَالَ أَ مَعَكَ أَحَدٌ قَالَ لَا قَالَ لَا تَكُنْ وَحْدَكَ تَحَوَّلْ عَنْهُ يَا مَيْمُونُ- فَإِنَّ الشَّيْطَانَ أَجْرَأُ مَا يَكُونُ عَلَى الْإِنْسَانِ إِذَا كَانَ وَحْدَهُ.

📚 Al-Kafi, Vol.6, p.534

Imam Baqir (peace be upon him) said to Muhammad ibn Sulayman: Where are you staying? He said: In such and such place! Imam asked: Is anyone with you? He said: No! Imam said: Do not be alone! O monkey! Move from this place to another, because Satan has the greatest audacity over a person when they are alone!

✅In addition to the discouragement of sleeping alone, explained in Question 475, being alone in a room or alone in a house is prohibited, even if one is engaged in prayer!

اللَّهُمَّ فَصَلِّ عَلَی مُحَمَّدٍ وَ آلِ مُحَمَّدٍ وَ عَجِّلْ لِوَلِیِّکَ الْفَرَجَ وَ الْعَافِیَةَ وَ النَّصْرَ