Hadith #2333 (Sunan Abi Dawud)
Book: Fasting (Kitab Al-Siyam)
Narrator(s): Abu Ishaq al-Sabay'ai' أبو إسحاق السبيعي عمرو بن عبد الله, Ibn Umm Maktum, Muhammad bin 'Abdullah bin Numayr محمد بن عبد الله بن نمير, Sulaiman bin Hayan al-Azdi سليمان بن حيان الأزدي
Grade: Sahih

That man and the people of his city should not fast as an atonement except that they know (for certain) that the people of a certain city of Muslims had fasted on Sunday. In that case they should keep fast as an atonement.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-2333/