I came to Abu Sa'id al-Khudri while he was giving his legal opinion to the people who bent down on him. So I waited to see hi when he was alone. When he became alone, I asked him about keeping fast while travelling. He said: we went out along with the Prophet ﷺ in Ramadan in the year of conquest of Mecca. The Messenger of Allah ﷺ fasted and we fasted until he reached a certain stage. He said: You have come near your enemy; the breaking of fast will bring you more strength. Then morning came when some of us fasted and other broke their fast. He (Abu Sa'id al-Khudri) said: We then proceeded and alighted at a stage. He said: You are going to attack your enemy tomorrow morning ; breaking the fast will bring you more strength ; so break your fast (i.e. do not keep fast). This resolution (of breaking the fast) took place (due to the announcement) from the Messenger of Allah ﷺ. Abu Sa'id said: Then I found myself keeping fast along with the Prophet ﷺ before and after that.  

أَتَيْتُ أَبَا سَعِيدٍ الْخُدْرِيَّ وَهُوَ يُفْتِي النَّاسَ وَهُمْ مُكِبُّونَ عَلَيْهِ فَانْتَظَرْتُ خَلْوَتَهُ فَلَمَّا خَلاَ سَأَلْتُهُ عَنْ صِيَامِ رَمَضَانَ فِي السَّفَرِ فَقَالَ خَرَجْنَا مَعَ النَّبِيِّ ﷺ فِي رَمَضَانَ عَامَ الْفَتْحِ فَكَانَ رَسُولُ اللَّهِ ﷺ يَصُومُ وَنَصُومُ حَتَّى بَلَغَ مَنْزِلاً مِنَ الْمَنَازِلِ فَقَالَ إِنَّكُمْ قَدْ دَنَوْتُمْ مِنْ عَدُوِّكُمْ وَالْفِطْرُ أَقْوَى لَكُمْ فَأَصْبَحْنَا مِنَّا الصَّائِمُ وَمِنَّا الْمُفْطِرُ قَالَ ثُمَّ سِرْنَا فَنَزَلْنَا مَنْزِلاً فَقَالَ إِنَّكُمْ تُصَبِّحُونَ عَدُوَّكُمْ وَالْفِطْرُ أَقْوَى لَكُمْ فَأَفْطِرُوا فَكَانَتْ عَزِيمَةً مِنْ رَسُولِ اللَّهِ ﷺ قَالَ أَبُو سَعِيدٍ ثُمَّ لَقَدْ رَأَيْتُنِي أَصُومُ مَعَ النَّبِيِّ ﷺ قَبْلَ ذَلِكَ وَبَعْدَ ذَلِكَ  

I came to Abu Sa'id al-Khudri (Allah be pleased with him) and he was surrounded (by people), and when they dispersed I said to him: I am not going to ask you about what these people were asking. I ask you about fasting on a journey. Upon this he said: We travelled with the Messenger of Allah ﷺ towards Mecca and we had been observing fast. We halted at a place. There the Messenger of Allah ﷺ said: You are nearing your enemy and breaking of fast would give you greater strength, and that was a concession (given to us). But some of us continued to observe the fast and some of us broke it. We then got down at another place and he (the Holy Prophet) said: You are going to encounter the enemy in the morning and breaking of the fast would give you strength, so break the fast. As it was a point of stress, so we broke the fast. But subsequently we saw ourselves observing the fast with the Messenger of Allah ﷺ on a journey. (Using translation from Muslim 1120)   

أَتَيْتُ أَبَا سَعِيدٍ وَهُوَ مَكْثُورٌ عَلَيْهِ فَلَمَّا تَفَرَّقَ النَّاسُ عَنْهُ قُلْتُ إِنِّي لَا أَسْأَلُكَ عَمَّا سَأَلَكَ هَؤُلَاءِ أَسْأَلُكَ عَنِ الصَّوْمِ فِي السَّفَرِ فَقَالَ سَافَرْنَا مَعَ رَسُولِ اللهِ ﷺ إِلَى مَكَّةَ وَنَحْنُ صِيَامٌ فَنَزَلْنَا مَنْزِلًا فَقَالَ رَسُولُ اللهِ ﷺ إِنَّكُمْ قَدْ دَنَوْتُمْ مِنْ عَدُوِّكُمْ وَالْفِطْرُ أَقْوَى لَكُمْ فَكَانَتْ رُخْصَةً مِنَّا مَنْ صَامَ وَمِنَّا مَنْ أَفْطَرَ ثُمَّ نَزَلْنَا مَنْزِلًا آخَرَ فَقَالَ إِنَّكُمْ مُصَبِّحُوا عَدُوَّكُمْ وَالْفِطْرُ أَقْوَى لَكُمْ فَأَفْطِرُوا فَكَانَتْ عَزْمَةً فَأَفْطَرْنَا ثُمَّ قَالَ لَقَدْ رَأَيْتُنَا نَصُومُ مَعَ رَسُولِ اللهِ ﷺ بَعْدَ ذَلِكَ فِي السَّفَرِ