Messenger of Allah ﷺ met ʿUmar bin Al-Khattab while the latter was going with a group of camel-riders, and he was swearing by his father. The Prophet ﷺ said, "Lo! Allah forbids you to swear by your fathers, so whoever has to take an oath, he should swear by Allah or keep quiet."  

أَنَّ رَسُولَ اللَّهِ ﷺ أَدْرَكَ عُمَرَ بْنَ الْخَطَّابِ وَهْوَ يَسِيرُ فِي رَكْبٍ يَحْلِفُ بِأَبِيهِ فَقَالَ أَلاَ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ مَنْ كَانَ حَالِفًا فَلْيَحْلِفْ بِاللَّهِ أَوْ لِيَصْمُتْ  

I heard ʿUmar saying, "Messenger of Allah ﷺ said to me, 'Allah forbids you to swear by your fathers." ʿUmar said, "By Allah! Since I heard that from the Prophet ﷺ , I have not taken such an oath, neither intentionally, nor by reporting the oath of someone else."  

لِي رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ يَنْهَاكُمْ أَنْ تَحْلِفُوا بِآبَائِكُمْ قَالَ عُمَرُ فَوَاللَّهِ مَا حَلَفْتُ بِهَا مُنْذُ سَمِعْتُ النَّبِيَّ ﷺ ذَاكِرًا وَلاَ آثِرًا قَالَ مُجَاهِدٌ {أَوْ أَثَرَةٍ مِنْ عِلْمٍ} يَأْثُرُ عِلْمًا  

Messenger of Allah ﷺ said, "Do not swear by your fathers."  

رَسُولُ اللَّهُ ﷺ لاَ تَحْلِفُوا بِآبَائِكُمْ  

There was a relation of love and brotherhood between this tribe of Jarm and Al-Ashʿariyin. Once we were with Abu Musa Al-Ashʿari, and then a meal containing chicken was brought to Abu Musa, and there was present, a man from the tribe of Taimillah who was of red complexion as if he were from non-Arab freed slaves. Abu Musa invited him to the meal. He said, "I have seen chickens eating dirty things, so I deemed it filthy and took an oath that I would never eat chicken." On that, Abu Musa said, "Get up, I will narrate to you about that. Once a group of the Ashʿariyin and I went to Messenger of Allah ﷺ and asked him to provide us with mounts; he said, 'By Allah, I will never give you any mounts nor do I have anything to mount you on.' Then a few camels of war booty were brought to Messenger of Allah ﷺ , and he asked about us, saying, 'Where are the Ash-'ariyin?' He then ordered five nice camels to be given to us, and when we had departed, we said, 'What have we done? Messenger of Allah ﷺ had taken the oath not to give us any mounts, and that he had nothing to mount us on, and later he gave us that we might ride? Did we take advantage of the fact that Messenger of Allah ﷺ had forgotten his oath? By Allah, we will never succeed.' So we went back to him and said to him, 'We came to you to give us mounts, and you took an oath that you would not give us any mounts and that you had nothing to mount us on.' On that he said, 'I did not provide you with mounts, but Allah did. By Allah, if I take an oath to do something, and then find something else better than it, I do that which is better and make expiation for the dissolution of the oath.' "  

كَانَ بَيْنَ هَذَا الْحَىِّ مِنْ جَرْمٍ وَبَيْنَ الأَشْعَرِيِّينَ وُدٌّ وَإِخَاءٌ فَكُنَّا عِنْدَ أَبِي مُوسَى الأَشْعَرِيِّ فَقُرِّبَ إِلَيْهِ طَعَامٌ فِيهِ لَحْمُ دَجَاجٍ وَعِنْدَهُ رَجُلٌ مِنْ بَنِي تَيْمِ اللَّهِ أَحْمَرُ كَأَنَّهُ مِنَ الْمَوَالِي فَدَعَاهُ إِلَى الطَّعَامِ فَقَالَ إِنِّي رَأَيْتُهُ يَأْكُلُ شَيْئًا فَقَذِرْتُهُ فَحَلَفْتُ أَنْ لاَ آكُلَهُ فَقَالَ قُمْ فَلأُحَدِّثَنَّكَ عَنْ ذَاكَ إِنِّي أَتَيْتُ رَسُولَ اللَّهِ ﷺ فِي نَفَرٍ مِنَ الأَشْعَرِيِّينَ نَسْتَحْمِلُهُ فَقَالَ وَاللَّهِ لاَ أَحْمِلُكُمْ وَمَا عِنْدِي مَا أَحْمِلُكُمْ فَأُتِيَ رَسُولُ اللَّهِ ﷺ بِنَهْبِ إِبِلٍ فَسَأَلَ عَنَّا فَقَالَ أَيْنَ النَّفَرُ الأَشْعَرِيُّونَ فَأَمَرَ لَنَا بِخَمْسِ ذَوْدٍ غُرِّ الذُّرَى فَلَمَّا انْطَلَقْنَا قُلْنَا مَا صَنَعْنَا حَلَفَ رَسُولُ اللَّهِ ﷺ لاَ يَحْمِلُنَا وَمَا عِنْدَهُ مَا يَحْمِلُنَا ثُمَّ حَمَلَنَا تَغَفَّلْنَا رَسُولَ اللَّهِ ﷺ يَمِينَهُ وَاللَّهِ لاَ نُفْلِحُ أَبَدًا فَرَجَعْنَا إِلَيْهِ فَقُلْنَا لَهُ إِنَّا أَتَيْنَاكَ لِتَحْمِلَنَا فَحَلَفْتَ أَنْ لاَ تَحْمِلَنَا وَمَا عِنْدَكَ مَا تَحْمِلُنَا فَقَالَ إِنِّي لَسْتُ أَنَا حَمَلْتُكُمْ وَلَكِنَّ اللَّهَ حَمَلَكُمْ وَاللَّهِ لاَ أَحْلِفُ عَلَى يَمِينٍ فَأَرَى غَيْرَهَا خَيْرًا مِنْهَا إِلاَّ أَتَيْتُ الَّذِي هُوَ خَيْرٌ وَتَحَلَّلْتُهَا