Musnad Ahmad 116
It was narrated that Ibn 'Abbas said:
'Umar said: We were with the Messenger of Allah ﷺ on a journey and a man said: No, by my father. Another man said: “Do not swear by your fathers.` I turned and saw that it was the Messenger of Allah ﷺ.
حَدَّثَنَا أَبُو سَعِيدٍ، مَوْلَى بَنِي هَاشِمٍ قَالَ حَدَّثَنَا زَائِدَةُ، حَدَّثَنَا سِمَاكٌ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ قَالَ عُمَرُ كُنَّا مَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فِي رَكْبٍ فَقَالَ رَجُلٌ لَا وَأَبِي فَقَالَ رَجُلٌ لَا تَحْلِفُوا بِآبَائِكُمْ فَالْتَفَتُّ فَإِذَا هُوَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ‏.‏