Sunan Abi Dawud 24242
In-book no 3738حَدَّثَنَا 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.