Al-Mundhiri recorded the following chapter in his book Al-Targhib wal-Tarhib:

 ترهيب الصائم من الغيبة والفحش والكذب ونحو ذلك

Encouragement to fast from backbiting, obscenity, falsehood, and so on.

Source: al-Targhīb wal-Tarhīb 2/146

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

مَنْ لَمْ يَدَعْ قَوْلَ الزُّورِ وَالْعَمَلَ بِهِ فَلَيْسَ لِلَّهِ حَاجَةٌ فِي أَنْ يَدَعَ طَعَامَهُ وَشَرَابَهُ

Whoever does not leave evil words and deeds while fasting, Allah does not need him to leave food and drink.

Source: Ṣaḥīḥ al-Bukhārī 1903, Grade: Sahih

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

إِنَّ الصِّيَامَ لَيْسَ مِنَ الأَكْلِ وَالشُّرْبِ فَقَطْ إِنَّمَا الصِّيَامُ مِنَ اللَّغُوِ وَالرَّفَثِ فَإِنْ سَابَّكَ أَحَدٌ أَوْ جَهِلَ عَلَيْكَ فَقُلْ إِنِّي صَائِمٌ

Verily, fasting is not only from eating and drinking. Rather, fasting is from vanity and obscenity. If someone abuses you or acts foolish against you, then say: Indeed, I am fasting.

Source: Ṣaḥīḥ Ibn Ḥibbān 3561, Grade: Sahih

In another narration, the Prophet (ṣ) said:

لَا تُسَابَّ وَأَنْتَ صَائِمٌ فَإِنْ سَابَّكَ أَحَدٌ فَقُلْ إِنِّي صَائِمٌ وَإِنْ كُنْتَ قَائِمًا فَاجْلِسْ

Do not insult anyone while you are fasting. If anyone insults you, then say: Indeed, I am fasting. If you are standing, then sit down.

Source: Ṣaḥīḥ Ibn Khuzaymah 1994, Grade: Sahih

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

رُبَّ صَائِمٍ لَيْسَ لَهُ مِنْ صِيَامِهِ إِلَّا الْجُوعُ وَرُبَّ قَائِمٍ لَيْسَ لَهُ مِنْ قِيَامِهِ إِلَّا السَّهَرُ

One might fast and he gets nothing from his fast but hunger. One might pray at night but he gets nothing from his prayer but fatigue.

Source: Sunan Ibn Mājah 1690, Grade: Sahih

Ibn Umar reported: The Messenger of Allah, peace and blessings be upon him, said:

رُبَّ صَائِمٍ حَظُّهُ مِنْ صِيَامِهِ الْجُوعُ وَالْعَطَشُ وَرُبَّ قَائِمٍ حَظُّهُ مِنْ قِيَامِهِ السَّهَرُ

One who fasts might only acquire hunger and thirst from his fasting. One who stands for prayer might only acquire fatigue from his prayer.

Source: al-Mu’jam al-Kabīr 13412, Grade: Sahih