The Prophet ﷺ said, "When one of you is invited to a meal while he is fasting, let him say, 'I am fasting.'"Ubayy said, "We did not call him by the kunya Abu Zinad. We used to call him by the kunya Abu Abdul-Rahman."
حدثنا سفيان عن أبي الزناد عن الأعرج
يبلغ به النبي ﷺ إذا دعي أحدكم إلى طعام وهو صائم فليقل إني صائم قال أبي لم نكن نكنه بأبي الزناد كنا نكنيه بأبي عبد الرحمن
Its isnad is sahih. Muslim (1:316) and Abu Dawud (2461, 2:307, Awn al-Mabud) both narrated it, both through the route of Sufyan bin Uyayna, with this isnad.Munziri said (2351), Muslim, Tirmidhi, Nasai, and Ibn Majah also reported it. In H, it is traced back to the Prophet. The word "to" is not present in M, but it appears that it was present in some copies, and that is why it was written in K, then struck out for deletion, so we removed it.As for the word Ahmad, which his son narrated from him following the hadith, by it he means that Abu Zinad is not the kunya of Abdullah bin Zakwan, but rather it is a nickname for him. As for his actual kunya, it is Abu Abdul-Rahman. Indeed, it is related in Tahdheeb, from the narration of Ibn Uyayna from him, that he used to get angry at this nickname.