✅Answer: Pay attention to the following narration:

1 قَالَ أَبُو وَلَّادٍ الْحَنَّاطُ قُلْتُ لِأَبِی عَبْدِ اللَّهِ علیه السلام إِنِّی کُنْتُ نَوَیْتُ حِینَ دَخَلْتُ الْمَدِینَةَ أَنْ أُقِیمَ بِهَا عَشْراً فَأَتْمَمْتُ الصَّلَاةَ ثُمَّ بَدَا لِی أَنْ لَا أُقِیمَ بِهَا فَمَا تَرَی لِی أُتِمُّ أَمْ أَقْصُرُ فَقَالَ لِی إِنْ کُنْتَ دَخَلْتَ الْمَدِینَةَ وَ صَلَّیْتَ بِهَا صَلَاةً وَاحِدَةً فَرِیضَةً بِتَمَامٍ فَلَیْسَ لَکَ أَنْ تَقْصُرَ حَتَّی تَخْرُجَ مِنْهَا وَ إِنْ کُنْتَ حِینَ دَخَلْتَهَا عَلَی نِیَّتِکَ فِی التَّمَامِ وَ لَمْ تُصَلِّ فِیهَا صَلَاةً فَرِیضَةً وَاحِدَةً بِتَمَامٍ حَتَّی بَدَا لَکَ أَنْ لَا تُقِیمَ فَأَنْتَ فِی تِلْکَ الْحَالِ بِالْخِیَارِ إِنْ شِئْتَ فَانْوِ الْمُقَامَ عَشْراً وَ أَتِمَّ وَ إِنْ لَمْ تَنْوِ الْمُقَامَ عَشْراً فَقَصِّرْ مَا بَیْنَکَ وَ بَیْنَ شَهْرٍ فَإِذَا مَضَی لَکَ شَهْرٌ فَأَتِمَّ الصَّلَاةَ.

📚 Faqih, vol. 1, p. 437

Abu Walad al-Hannat said: I said to Imam Sadiq (peace be upon him): 'When I entered Medina, I intended to stay for ten days and I prayed my prayer in full. Then something happened to me, and it became clear to me that I would not stay for ten days. What do you think is correct? Should I pray my prayer in full or shortened?'

Imam Sadiq (peace be upon him) said: 'If you entered Medina and (because of your intention to stay for ten days) you prayed one obligatory prayer in full, then you are not to pray shortened until you leave this city. But if you entered the city with the intention of praying in full (and staying for ten days) and it became clear to you before you prayed one obligatory prayer in full that you would not stay, then you have a choice: either you intend to stay for ten days and pray the prayer in full, or if you do not intend to stay for ten days, you pray your prayer shortened for up to one month. And when your one month is complete, you pray the prayer in full.'

✅According to the above narration, if a person enters a city with the intention of staying for ten days, there is no doubt that his prayer is in full. However, sometimes after intending to stay for ten days, something happens that causes the person to change his mind and intend to stay for less than ten days. Now, if this change of decision occurs before he has prayed one prayer in full, from that point on, he should pray his prayers shortened. But if he has prayed at least one full prayer (Dhuhr, Asr, or 'Isha), with this change of decision, his prayer is not shortened, and for the entire duration of his stay in that city, he prays his prayers in full.

For example, a person enters the city of Mashhad with the intention of staying for ten days or more. Now, if before he prays one Dhuhr, Asr, or 'Isha prayer in full, he decides to stay for less than ten days, he prays his prayers shortened. But if he has prayed even one Dhuhr, Asr, or 'Isha prayer in full, even though his intention to stay for ten days has changed, he prays his prayers in full for the duration of his stay in that city.

✅In the above narration, the ruling on prayer is stated. To clarify the ruling on fasting, we use the following narration:

2 عَنْ سَمَاعَةَ قَالَ: قَالَ أَبُو عَبْدِ اللَّهِ فِي حَدِيثٍ وَ لَيْسَ يَفْتَرِقُ التَّقْصِيرُ وَ الْإِفْطَارُ فَمَنْ قَصَّرَ فَلْيُفْطِرْ.

📚 Tahdhib, vol. 4, p. 328

Imam Sadiq (peace be upon him) said in a hadith: 'The shortening of prayer and the breaking of the fast are not separate from each other, so whoever prays shortened must also break his fast.'

3 مُحَمَّدُ بْنُ عَلِيِّ بْنِ الْحُسَيْنِ بِإِسْنَادِهِ عَنْ مُعَاوِيَةَ بْنِ وَهْبٍ عَنْ أَبِي عَبْدِ اللَّهِ فِي حَدِيثٍ قَالَ: هَذَا وَاحِدٌ إِذَا قَصَرْتَ أَفْطَرْتَ وَ إِذَا أَفْطَرْتَ قَصَرْتَ.

📚 Al-Faqih, vol. 1, p. 437

Imam Sadiq (peace be upon him) said in a hadith: 'This is one thing: when you pray shortened, you also break your fast, and when you break your fast, you also pray shortened.'

✅According to the two above narrations, when the ruling on prayer is established, fasting follows it, and when the ruling on fasting is established, prayer also follows it. In the case of our question, this person who changes his intention to stay, if it is before he has prayed a full Dhuhr, Asr, or 'Isha prayer, during his stay in that city, his prayer is shortened and his fast is broken. And if, because of his intention to stay for ten days, he prayed one full prayer and then changed his mind, during his stay in that city, his prayer is full and he must also fast.

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