From what I gather from scholars, they refer to several hadeeths indicating the permissibility of this, albeit while warning it does not mean to make it the usual practice, as Rasullah SA never did so.
An example of such hadeeth is this one:
"It was narrated from Ibn 'Abbas that the Prophet (ﷺ) used to pray in Al-Madinah combining two prayer. Joining Zuhr and 'Asr, and Maghrib and 'Isha', when there was no fear nor rain. It was said to him:
"Why?" He said: "So that there would not be any hardship on his Ummah."
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الْعَزِيزِ بْنِ أَبِي رِزْمَةَ، - وَاسْمُهُ غَزْوَانُ - قَالَ حَدَّثَنَا الْفَضْلُ بْنُ مُوسَى، عَنِ الأَعْمَشِ، عَنْ حَبِيبِ بْنِ أَبِي ثَابِتٍ، عَنْ سَعِيدِ بْنِ جُبَيْرٍ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يُصَلِّي بِالْمَدِينَةِ يَجْمَعُ بَيْنَ الصَّلاَتَيْنِ بَيْنَ الظُّهْرِ وَالْعَصْرِ وَالْمَغْرِبِ وَالْعِشَاءِ مِنْ غَيْرِ خَوْفٍ وَلاَ مَطَرٍ . قِيلَ لَهُ لِمَ قَالَ لِئَلاَّ يَكُونَ عَلَى أُمَّتِهِ حَرَجٌ .
Grade: Sahih (Darussalam)
Reference : Sunan an-Nasa'i 602
In-book reference : Book 6, Hadith 109
English translation : Vol. 1, Book 6, Hadith 603" from the webpage
https://sunnah.com/nasai:602.
This further one, for instance, informs that what one understands as 'fear' can be understood broadly:
"Kathir bin Qarawanda said:
"We asked Salim bin 'Abdullah about prayer while traveling. We said: 'Did 'Abdullah combine any of his prayer while traveling?' He said: 'No, except at Jam'.'[1] Then he paused, and said: 'Safiyyah was married to him, and she sent word to him that she was in her last day in this world and the first day in the Hereafter. So he ride off in a hurry, and I was with him. The time for prayer came and the Mu'adhdhin said to him: 'The prayer, O Abu 'Abdur-Rahman! But he kept going until it was between the time for the two prayer. Then he stopped and said to the Mu'adhdhin: "Say the Iqamah, and when I say the Taslim at the end of Zuhr, say the Iqamah (again) straight away." So he said the Iqamah and he prayed Zuhr, two Rak'ahs, then he said the Iqamah (again) straight away, and he prayed 'Asr, two Rak'ahs. Then he rode off quickly until the sun set and the Mu'adhdhin said to him: "The prayer, O Abu 'Abdur-Rahman!" He said: "Do what you did before." He rode on until the starts appeared, then he stopped and said: "Say the Iqamah, then when I say the Taslim, say the Iqamah. So he said the Iqamah and he prayed Maghrib, three Rak'ahs, then he said the Iqamah (again) straight away and he prayed 'Isha', then he said one Taslim, turning his face. Then he said: "The Messenger of Allah (ﷺ) said: 'If any one of you has urgent need that he fears he may miss, let him pray like this.'
أَخْبَرَنَا عَبْدَةُ بْنُ عَبْدِ الرَّحِيمِ، قَالَ حَدَّثَنَا ابْنُ شُمَيْلٍ، قَالَ حَدَّثَنَا كَثِيرُ بْنُ قَارَوَنْدَا، قَالَ سَأَلْنَا سَالِمَ بْنَ عَبْدِ اللَّهِ عَنِ الصَّلاَةِ، فِي السَّفَرِ فَقُلْنَا أَكَانَ عَبْدُ اللَّهِ يَجْمَعُ بَيْنَ شَىْءٍ مِنَ الصَّلَوَاتِ فِي السَّفَرِ فَقَالَ لاَ إِلاَّ بِجَمْعٍ ثُمَّ أَتَيْتُهُ فَقَالَ كَانَتْ عِنْدَهُ صَفِيَّةُ فَأَرْسَلَتْ إِلَيْهِ أَنِّي فِي آخِرِ يَوْمٍ مِنَ الدُّنْيَا وَأَوَّلِ يَوْمٍ مِنَ الآخِرَةِ . فَرَكِبَ وَأَنَا مَعَهُ فَأَسْرَعَ السَّيْرَ حَتَّى حَانَتِ الصَّلاَةُ فَقَالَ لَهُ الْمُؤَذِّنُ الصَّلاَةَ يَا أَبَا عَبْدِ الرَّحْمَنِ . فَسَارَ حَتَّى إِذَا كَانَ بَيْنَ الصَّلاَتَيْنِ نَزَلَ فَقَالَ لِلْمُؤَذِّنِ أَقِمْ فَإِذَا سَلَّمْتُ مِنَ الظُّهْرِ فَأَقِمْ مَكَانَكَ . فَأَقَامَ فَصَلَّى الظُّهْرَ رَكْعَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ أَقَامَ مَكَانَهُ فَصَلَّى الْعَصْرَ رَكْعَتَيْنِ ثُمَّ رَكِبَ فَأَسْرَعَ السَّيْرَ حَتَّى غَابَتِ الشَّمْسُ فَقَالَ لَهُ الْمُؤَذِّنُ الصَّلاَةَ يَا أَبَا عَبْدِ الرَّحْمَنِ . فَقَالَ كَفِعْلِكَ الأَوَّلِ . فَسَارَ حَتَّى إِذَا اشْتَبَكَتِ النُّجُومُ نَزَلَ فَقَالَ أَقِمْ فَإِذَا سَلَّمْتُ فَأَقِمْ . فَصَلَّى الْمَغْرِبَ ثَلاَثًا ثُمَّ أَقَامَ مَكَانَهُ فَصَلَّى الْعِشَاءَ الآخِرَةَ ثُمَّ سَلَّمَ وَاحِدَةً تِلْقَاءَ وَجْهِهِ ثُمَّ قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا حَضَرَ أَحَدَكُمْ أَمْرٌ يَخْشَى فَوْتَهُ فَلْيُصَلِّ هَذِهِ الصَّلاَةَ " .
Grade: Sahih (Darussalam)
Reference : Sunan an-Nasa'i 597
In-book reference : Book 6, Hadith 104
English translation : Vol. 1, Book 6, Hadith 598" from the webpage https://sunnah.com/nasai:597.
May this help.
Allah knows best.