✅ Answer: Please pay attention to the following narrations:

1 معاوية بن وهب قال: قُلْتُ لِأَبِي عَبْداللَّه أَدْنَى مَا يُقَصِّرُ فِيهِ الْمُسَافِرُ الصَّلَاةَ قَالَ بَرِيدٌ ذَاهِباً وَ بَرِيدٌ جَائِياً

📚 Tahdhib Vol. 4, p. 224

Mu'awiyah ibn Wahb said: I said to Imam Sadiq (peace be upon him): What is the minimum distance for a traveler to shorten the prayer? He said: One 'barid' (i.e., 4 parasangs) going and one 'barid' returning!

2 عن أَبِي عَبْداللَّه قَال: فِي التَّقْصِيرِ حَدُّهُ أَرْبَعَةٌ وَ عِشْرُونَ مِيلًا

📚 Al-Tahdhib Vol. 4, p. 221

Imam Sadiq (peace be upon him) said regarding the shortening of prayer: Its limit is 24 miles!

3 عن الرِّضَا قال: و التَّقْصِيرُ فِي ثَمَانِيَةِ فَرَاسِخَ وَ مَا زَادَ وَ إِذَا قَصَّرْتَ أَفْطَرْتَ

📚 Ilal al-Shara'i p. 266, hadith 9

Imam Reza (peace be upon him) said: The shortening of prayer is at 8 parasangs and more, and when you shorten the prayer, you also break the fast!

4 عن أَبي عبداللَّه قال: بَيْنَا نَحْنُ جُلُوسٌ و أَبِي عِنْدَ وَالٍ لِبَنِي أُمَيَّةَ عَلَى الْمَدِينَة إِذْ جَاءَ أَبِي فَجَلَسَ فقال كُنْتُ عِنْدَ هَذَا قُبَيْلُ فَسَاءَلَهُمْ عَنِ التَّقْصِيرِ فقال قَائِلٌ مِنْهُمْ فِي ثَلَاثٍ و قال قَائِلٌ مِنْهُمْ يَوْمٍ و لَيْلَةٍ و قال قَائِلٌ مِنْهُمْ رَوْحَةٍ فَسَأَلَنِي فَقُلْتُ لَهُ إِنَ رَسُولَ اللَّه لَمَّا نَزَلَ عَلَيْهِ جَبْرَئِيلُ بِالتَّقْصِيرِ قَالَ لَهُ النَّبِيُّ في كَمْ ذَاكَ فَقَالَ في بَرِيدٍ قَالَ و أَيُّ شَيْءٍ الْبَرِيدُ فَقَالَ مَا بَيْنَ ظِلِّ عَيْرٍ إِلَى فَيْءِ وُعَيْرٍ قال ثُمَّ عَبَرْنَا زَمَاناً ثُمَّ رَأَى [رُؤِيَ] بَنُو أُمَيَّةَ يَعْمَلُونَ أَعْلَاماً عَلَى الطَّرِيقِ وَ أَنَّهُمْ ذَكَرُوا مَا تَكَلَّمَ بِهِ أَبُو جَعْفَرٍ فَذَرَعُوا ما بَيْنَ ظِلِّ عَيْرٍ إِلَى فَيْءِ وُعَيْرٍ ثُمَّ جَزَّءُوهُ عَلَى اثْنَيْ عَشَرَ مِيلًا فَكَانَتْ ثَلَاثَةَ آلَافٍ وَ خَمْسَمِائَةِ ذِرَاعٍ كُلُّ مِيلٍ فَوَضَعُوا الْأَعْلَامَ فَلَمَّا ظَهَرَ بَنُو هَاشِمٍ غَيَّرُوا أَمْرَ بَنِي أُمَيَّةَ غَيْرَةً لِأَنَّ الْحَدِيثَ هَاشِمِيٌّ فَوَضَعُوا إِلَى جَنْبِ كُلِّ عَلَمٍ عَلَماً

📚 Kafi Vol. 3, p. 432

Imam Sadiq (peace be upon him) said: While we were sitting and my father was with one of the Umayyad rulers over Medina, my father suddenly came and sat down and said: I was with this (Umayyad ruler) a little while ago, and he asked them about the shortening of prayer;

So one of them said: In three days (of travel), and one said: One day and one night, and one said: From morning until night!

Then he asked me, and I said to him: When Gabriel brought the ruling of the shortened prayer to the Messenger of Allah (peace be upon him and his family), he said to him: At what distance? Gabriel said: At one 'barid'!

He said to him: What is a 'barid'? Gabriel said: The distance between the shadow of Mount 'Ayr and the shadow of Mount Wu'ayr!

He said: Then we spent some time until it was seen that the Umayyads were setting up signs on the road, and they were speaking about what Imam Baqir (peace be upon him) had said!

So they measured the distance between the shadow of Mount 'Ayr and the shadow of Mount Wu'ayr with the unit of a 'dhira' and divided it by twelve miles, so each mile was 3500 'dhira', so they set up signs for it!

So when the Banu Hashim prevailed, they changed the order of the Umayyads because the hadith was Hashimi, and therefore they set up a sign next to each sign!

5 Zurarah said: I asked Imam Baqir (peace be upon him) about the time of the Dhuhr prayer; he said: One 'dhira' from the time of the sun's decline, and the time of the 'Asr prayer is one 'dhira' from the time of Dhuhr, which is four steps from the time of the sun's decline... (meaning that two 'dhira' are equivalent to four steps)

📚 Faqih Vol. 1, p. 217

✅ It is clear from the above narrations and others that the religious distance for shortening the prayer and breaking the fast is 8 parasangs or two 'barid' or 24 miles, and this distance can be 4 parasangs going and 4 parasangs returning, and therefore, for a person whose journey is at least 8 parasangs, even if it is 4 parasangs going and 4 parasangs returning, his prayer is shortened and his fast is broken.

✅ It is clear from narration 4️⃣ and others that each mile is equivalent to 3500 'dhira', and one of the famous meanings of a 'dhira' is the distance from the elbow to the fingertips, which in narration 5️⃣ is equivalent to two steps, and although its amount varies slightly in different people, the usual measure of each step is about 26 to 27 cm, and consequently, each 'dhira' is about 52 to 54 cm, and following that, four parasangs is about 21.84 to 22.68 km, and 8 parasangs is about 43.68 to 45.36 km, and at least at such a distance from the end of each city, whether it is 4 parasangs going and 4 parasangs returning or a continuous journey, the prayer and fast are shortened.