Sheikh Salih Al-Luhaidan rahimahullah says:

It was narrated in a hadith that the Prophet ﷺ said: ((لا يُصَلِّيَنَّ أَحَدُكُمْ فِي الثَّوْبِ الْوَاحِدِ لَيْسَ عَلَى عَاتِقِهِ مِنْهُ شَىْءٌ)) "None of you should pray in a single garment that does not cover his shoulders." Thus, someone wearing only a lower garment and praying, for example, a voluntary prayer at home without covering their shoulders, may take the matter lightly. While such a prayer is valid, one should make every effort to prepare appropriately for prayer as much as possible. Allāh says: ﴿یَـٰبَنِیۤ ءَادَمَ خُذُوا۟ زِینَتَكُمۡ عِندَ كُلِّ مَسۡجِدࣲ﴾ "O children of Ādam, take your adornment at every place of prayer." [Surah Al-Aʿrāf: 31] This means one should strive to dress properly for every prayer. If one cannot wear their finest attire, they should at least ensure that their body is fully covered. It is not appropriate to say, "I have covered from the navel to the knees, so the rest doesn’t matter." No! If one is going to meet other people they would not want to meet them looking dishevelled. Therefore, since prayer is one will stand before their Lord in prayer, at the very least, one should wear clothing that adequately covers the entire body. `Umar ibn Abi Salamah, the stepson of the Prophet ﷺ, narrated: "I saw the Prophet praying in a single garment." When the Prophet ﷺ was asked about this, he replied, "Does each one of you have two garments?" At that time, most people possessed only one garment. Similarly, a woman in her menstrual period would use her single garment despite the blood stains. After her purification, she would clean the bloodied area by scrubbing it and washing it, even if the stain’s colour remained, as she had no other garment. This reflects the simplicity and scarcity of possessions during the Prophet’s time. In contrast, even the poorest individuals in some countries today may have greater material means than the wealthier individuals of that era. Back then, the majority of people owned only one piece of clothing.