✅Answer:

1 عَنْ أَبِي عَبْداللَّه قَالَ: لَا تَمْشِ فِي حِذَاءٍ وَاحِدٍ قُلْتُ وَ لِمَ قَالَ لِأَنَّهُ إِنْ أَصَابَكَ مَسٌّ مِنَ الشَّيْطَانِ لَمْ يَكَدْ يُفَارِقُكَ إِلَّا مَا شَاءَ اللَّهُ

📚Al-Kafi, Vol. 6, p. 467

Halabi said: Imam Sadiq (peace be upon him) said: 'Do not walk in a single sandal!' I said: 'Why?!' He said: 'Because if a madness from Satan reaches you, it will not leave you unless Allah wills!'

2 عَنْ أَبِی الْحَسَن مُوسَی قَالَ: ثَلَاثَةٌ یُتَخَوَّفُ مِنْهَا الْجُنُونُ التَّغَوُّطُ بَیْنَ الْقُبُورِ وَ الْمَشْیُ فِی خُفٍّ وَاحِدٍ وَ الرَّجُلُ یَنَامُ وَحْدَهُ

📚Al-Kafi, Vol. 6, p. 234

Imam Kazim (peace be upon him) said: 'There are three things from which one is feared to go mad: defecating among graves, walking in a single shoe, and a person sleeping alone!'

(Defecating: 'Gha'it')

3 'An Muhammad ibn Muslim 'an ahadihima anna-hu qala la tashrab wa anta qa'im wa la tabul fi ma'in naqi' wa la tataff 'ala qabr wa la takhlu fi baytin wahdaka wa la tamshi fi na'lin wahida fa-inna al-shaytan asra'u ma yakunu ila al-'abd idha kana 'ala ba'di hadhihi al-ahwal wa qala innahu ma asaba ahadan shay' 'ala hadhihi al-hal fakada an yufariqahu illa yasha' Allah

📚Al-Kafi, Vol. 6, p. 234

Muhammad ibn Muslim narrated from one of the two Imams, Sadiq or Baqir (peace be upon them), that they said: 'Do not drink while standing, and do not urinate in stagnant water, and do not defecate on a grave, and do not be alone in a room, and do not walk in a single sandal, for Satan is fastest to the servant when he is in one of these states, and he said: Nothing befalls anyone in this state that is about to leave him unless Allah, the Mighty and Exalted, wills!'

4 عَنِ الصَّادِقِ عَنْ آبَائِهِ فِي حَدِيثِ الْمَنَاهِي قَالَ : نَهَى رَسُولُ اللَّهِ أَنْ يَمْشِيَ الرَّجُلُ فِي فَرْدِ نَعْلٍ وَ أَنْ يَتَنَعَّلَ وَ هُوَ قَائِمٌ

📚Al-Faqih, Vol. 4, p. 4

Imam Sadiq (peace be upon him) from his ancestors from the Messenger of Allah (peace be upon him and his family) in the hadith of prohibitions, forbade a person from walking in a single sandal, and also forbade him from putting on sandals while standing!

5 Ya'qub al-Sarraj qala kunna namshi ma'a Abi 'Abdillah wa huwa yuridu an yu'azzi dha qaraba lahu bi-mawludin lahu fa-inqata'a shas'u na'li Abi 'Abdillah fa-tanawala na'lahu min rijlihi thumma masha hafiyan fa-nazara ilayhi ibn Abi Ya'fur fa-khala'a na'la nafsihi min rijlihi wa khala'a al-shas' min-ha wa nawalahu Aba 'Abdillah fa-a'rada 'anhu ka-hay'at al-mughdab thumma aba an yaqbalahu thumma qala ala inna sahib al-masiba awla bi-al-sabr 'alayha fa-masha hafiyan hatta dakhala 'ala al-rajul alladhi atahu li-yu'azzih

📚Al-Kafi, Vol. 6, p. 464

Ya'qub al-Sarraj said: We were walking with Imam Sadiq (peace be upon him), and the Imam intended to offer condolences to one of his relatives for the loss of his child, so the strap of Imam Sadiq's (peace be upon him) sandal broke. The Imam took his sandal off his foot and walked barefoot!

Abdullah ibn Abi Ya'fur saw the Imam and took his own sandal off his foot and took out its strap and gave it to the Imam (peace be upon him)!

The Imam turned away from him as if he were angry and refused to accept it!

Then he said: 'Indeed, the one afflicted with a calamity is more worthy of patience with it,' so he walked barefoot until he entered upon the man he had come to offer condolences to!

6 عَنْ أَبِي عَبْداللَّه عَنْ عَلِيٍّ أَنَّهُ كَانَ يَمْشِي فِي نَعْلٍ وَاحِدَةٍ وَ يُصْلِحُ الْأُخْرَى لَا يَرَى بِذَلِكَ بَأْساً

📚Al-Kafi, Vol. 6, p. 468

Imam Sadiq (peace be upon him) said: Ali (peace be upon him) used to walk in a single sandal while he was mending and fixing the other sandal and he saw no problem with this!

7 مُحَمَّدُ بْنُ عَلِيِّ بْنِ الْحُسَيْنِ قَالَ رُوِيَ... وَ الْحَاجُّ إِذَا انْقَطَعَ شِسْعُ نَعْلِهِ كَتَبَ اللَّهُ لَهُ ثَوَابَ مَا بَيْنَ مَشْيِهِ حَافِياً إِلَى مُنْتَعِلٍ

📚Al-Faqih, Vol. 2, p. 218

Sheikh Saduq said: It has been narrated: 'When the strap of a pilgrim's sandal breaks, Allah writes for him the reward of his walking during the time between walking barefoot and walking with a sandal!'

✅It is clear from the above narrations and others that walking in a single sandal, which is footwear with a sole and two straps (something similar to today's flip-flops), and walking in a single 'hida' (a type of sandal), and walking in a single 'khuff' (a type of footwear that covers the top of the foot) is forbidden, and there is a fear for the one who does this that a madness from Satan may befall him and not leave him unless Allah, the Mighty and Exalted, wills.

However, in a situation where a person's sandal has broken and he has taken it off to fix it, walking in a single sandal is permitted.