Musnad Ahmad 140
It was narrated that ‘Umar said:
We went on a campaign with the Messenger of Allah ﷺ during Ramadan and the conquest took place in Ramadan, and we broke the fast in both cases.
حَدَّثَنَا أَبُو سَعِيدٍ، حَدَّثَنَا ابْنُ لَهِيعَةَ، حَدَّثَنَا بُكَيْرٌ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ عُمَرَ، قَالَ غَزَوْنَا مَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي رَمَضَانَ وَالْفَتْحَ فِي رَمَضَانَ فَأَفْطَرْنَا فِيهِمَا‏.‏