✅ Answer: First, see the explanation of the 'day of doubt' in the following link, and then pay attention to the narrations below:

https://feghh14.com/en/623

1 عَنْ عَبْدِ الرَّحْمَنِ بْنِ أَبِي عَبْدِ اللَّهِ قَالَ: سَأَلْتُ أَبَا عَبْدِ اللَّهِ عَنْ هِلَالِ رَمَضَانَ يُغَمُّ عَلَيْنَا فِي تِسْعٍ وَ عِشْرِينَ مِنْ شَعْبَانَ فَقَالَ لَا تَصُمْ إِلَّا أَنْ تَرَاهُ فَإِنْ شَهِدَ أَهْلُ بَلَدٍ آخَرَ فَاقْضِهِ

📚 Al-Tahdhib, Vol. 4, p. 157

Abd al-Rahman said: I asked Imam Sadiq (peace be upon him) about the crescent of Ramadan when it is cloudy and hidden from us on the 29th of Sha'ban. The Imam said: 'Do not fast (as Ramadan) unless you see the crescent. And if the people of another city later testify (that they saw the crescent of Ramadan on the evening of the 29th of Sha'ban), then make it up!'

2 عَنْ سَمَاعَةَ أَنَّهُ سَأَلَ أَبَا عَبْدِ اللَّهِ عَنِ الْيَوْمِ فِي شَهْرِ رَمَضَانَ يُخْتَلَفُ فِيهِ قَالَ إِذَا اجْتَمَعَ أَهْلُ مِصْرٍ عَلَى صِيَامِهِ لِلرُّؤْيَةِ فَاقْضِهِ إِذَا كَانَ أَهْلُ الْمِصْرِ خَمْسَمِائَةِ إِنْسَانٍ.

📚 Al-Faqih, Vol. 2, p. 124

Sam'ah asked Imam Sadiq (peace be upon him) about the day when there is a dispute regarding the month of Ramadan (i.e., the 'day of doubt'). The Imam said: 'If the people of a city agree on fasting it based on a sighting, then make it up, provided that the people of that city are 500 people!'

3 عَنْ مَنْصُورِ بْنِ حَازِمٍ عَنْ أَبِي عَبْدِ اللَّهِ أَنَّهُ قَالَ: صُمْ لِرُؤْيَةِ الْهِلَالِ وَ أَفْطِرْ لِرُؤْيَتِهِ فَإِنْ شَهِدَ عِنْدَكَ شَاهِدَانِ مَرْضِيَّانِ بِأَنَّهُمَا رَأَيَاهُ فَاقْضِهِ.

📚 Al-Tahdhib, Vol. 4, p. 157

Imam Sadiq (peace be upon him) said: 'Fast based on the sighting of the crescent and break your fast based on its sighting. So if two trustworthy witnesses testify that they saw the crescent (earlier), then make it up!'

4 عبد اللّه بن علي الحلبي عَنْ أَبِي عَبْدِ اللَّهِ فِي حَدِيثٍ قَالَ: قُلْتُ أَ رَأَيْتَ إِنْ كَانَ الشَّهْرُ تِسْعَةً وَ عِشْرِينَ يَوْماً أَقْضِي ذَلِكَ الْيَوْمَ فَقَالَ لَا إِلَّا أَنْ يَشْهَدَ لَكَ بَيِّنَةٌ عُدُولٌ فَإِنْ شَهِدُوا أَنَّهُمْ رَأَوُا الْهِلَالَ قَبْلَ ذَلِكَ فَاقْضِ ذَلِكَ الْيَوْمَ

📚 Al-Tahdhib, Vol. 4, p. 156

Halabi said: I said to Imam Sadiq (peace be upon him): 'Inform me, if Ramadan is 29 days, should I make up that one day?' He said: 'No! Unless just witnesses testify to you! So if they testify that they saw the crescent before that, then make up that one day!'

5 عَنْ هِشَامِ بْنِ الْحَكَمِ عَنْ أَبِي عَبْدِ اللَّه أَنَّهُ قَالَ: فِيمَنْ صَامَ تِسْعَةً وَ عِشْرِينَ قَالَ إِنْ كَانَتْ لَهُ بَيِّنَةٌ عَادِلَةٌ عَلَى أَهْلِ مِصْرٍ أَنَّهُمْ صَامُوا ثَلَاثِينَ عَلَى رُؤْيَةٍ قَضَى يَوْماً.

📚 Al-Tahdhib, Vol. 4, p. 158

Imam Sadiq (peace be upon him) said about a person who has fasted 29 days: 'If there is a just witness for the people of a city that they have fasted 30 days based on a sighting, he makes up one day!'

6 عَنْ يُونُسَ بْنِ يَعْقُوبَ قَالَ: قُلْتُ لِأَبِي عَبْدِ اللَّهِ إِنِّي صُمْتُ شَهْرَ رَمَضَانَ عَلَى رُؤْيَةٍ تِسْعَةً وَ عِشْرِينَ يَوْماً وَ مَا قَضَيْتُ قَالَ فَقَالَ وَ أَنَا قَدْ صُمْتُهُ وَ مَا قَضَيْتُ

📚 Al-Tahdhib, Vol. 4, p. 160

Yunus ibn Ya'qub said: I said to Imam Sadiq (peace be upon him): 'I fasted Ramadan for 29 days based on a sighting and I did not make it up!' The Imam said: 'I also fasted in the same way and did not make it up!'

7 عَبْداللَّه بْنِ سِنَان عَنْ رَجُلٍ نَسِيَ حَمَّادُ بْنُ عِيسَى اسْمَهُ قال: صَامَ عَلِيٌّ بِالْكُوفَةِ ثَمَانِيَةً وَ عِشْرِينَ يَوْماً شَهْرَ رَمَضَانَ فَرَأَوُا الْهِلَالَ فَأَمَرَ مُنَادِياً يُنَادِي اقْضُوا يَوْماً فَإِنَّ الشَّهْرَ تِسْعَةٌ وَ عِشْرُونَ يَوْماً

📚 Tahdhib, Vol. 4, p. 158

A man said: Ali (peace be upon him) in Kufa fasted Ramadan for 28 days, and then the people saw the crescent (of the month of Shawwal)!

Ali (peace be upon him) ordered a crier to announce: 'Make up one day, for Ramadan was 29 days!'

✅ It is clear from the above narrations and others that if a person did not fast on the 'day of doubt' or was traveling and fasted Ramadan for 29 days based on the sighting of the crescent of Shawwal, he makes up one day in a few cases:

1. Just witnesses testify that they saw the crescent one day earlier.

2. It is proven by a just witness that the people of a city fasted for 30 days based on a sighting.

3. The crescent of Shawwal is sighted and they have fasted a total of 28 days.

✅ In other than the above cases, no instruction has been received regarding making up one day in a case where the month of Ramadan is 29 days based on a crescent sighting.