Al-Kāfi - Volume 4 > The Case of Those Who Need Permission to Fast Optional Fasting
Hadith #3

3ـ عَلِيُّ بْنُ مُحَمَّدِ بْنِ بُنْدَارَ وَغَيْرُهُ عَنْ إِبْرَاهِيمَ بْنِ إِسْحَاقَ بِإِسْنَادٍ ذَكَرَهُ عَنِ الْفُضَيْلِ بْنِ يَسَارٍ عَنْ أَبِي جَعْفَرٍ (عَلَيْهِ السَّلاَم) قَالَ قَالَ رَسُولُ الله (صلَّى اللهُ عَلَيْهِ وَآلِه) إِذَا دَخَلَ رَجُلٌ بَلْدَةً فَهُوَ ضَيْفٌ عَلَى مَنْ بِهَا مِنْ أَهْلِ دِينِهِ حَتَّى يَرْحَلَ عَنْهُمْ وَلا يَنْبَغِي لِلضَّيْفِ أَنْ يَصُومَ إِلاَّ بِإِذْنِهِمْ لِئَلاَّ يَعْمَلُوا الشَّيْ‏ءَ فَيَفْسُدَ عَلَيْهِمْ وَلا يَنْبَغِي لَهُمْ أَنْ يَصُومُوا إِلاَّ بِإِذْنِ الضَّيْفِ لِئَلاَّ يَحْتَشِمَهُمْ فَيَشْتَهِيَ الطَّعَامَ فَيَتْرُكَهُ لَهُمْ.

3. Ali ibn Muhammad ibn Bandar [and others] from Ibrahim ibn Ishaq through his chain of narrators from al-Fadl ibn Yasar who has said the following: “Abu Ja‘far, ‘Alayhi al-Salam, has stated this Hadith: ‘The Messenger of Allah, O Allah, grant compensation to Muhammad and his family worthy of their services to Your cause, has said, “When a man enters a town (residence), he is a guest of the inhabitants of that town who are followers of the religion he follows until he leaves that town (residence). It is not proper for him to fast without their permission, so that the food that may have been prepared does not spoil. It is not proper for them also to fast without permission so he because of exercising reservation does not eat the food that he desires.’””