Hadith 3737
Chapter 1: What has been reported about accepting invitations - كتاب الأطعمة
حَدَّثَنَا مَخْلَدُ بْنُ خَالِدٍ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم بِمَعْنَاهُ زَادَ " فَإِنْ كَانَ مُفْطِرًا فَلْيَطْعَمْ وَإِنْ كَانَ صَائِمًا فَلْيَدْعُ " .
The tradition mentioned above has also been transmitted by Ibn ‘Umar to the same effect through a different chain of narrators. This version has the additional words:
If he is not fasting, he should eat, and if he is fasting, he should leave it.
Reference : Sunan Abi Dawud 3737
In-book reference : Book 28, Hadith 2