When taking an oath, the Prophet ﷺ very often used to say, "No, by Him Who turns the hearts."  

كَثِيرًا مِمَّا كَانَ النَّبِيُّ ﷺ يَحْلِفُ (لاَ وَمُقَلِّبِ الْقُلُوبِ)  

The Prophet ﷺ said to Ibn Saiyad, "I have kept for you a secret." Ibn Saiyad said, "Ad-Dukh." The Prophet said, "Keep quiet, for you cannot go beyond your limits (or you cannot exceed what has been foreordained for you)." On that, ʿUmar said (to the Prophet ﷺ ), "Allow me to chop off his neck!" The Prophet said, "Leave him, for if he is he (i.e., Ad-Dajjal), then you will not be able to overcome him, and if he is not, then you gain no good by killing him."  

قَالَ النَّبِيُّ ﷺ لاِبْنِ صَيَّادٍ خَبَأْتُ لَكَ خَبِيئًا قَالَ الدُّخُّ قَالَ اخْسَأْ فَلَنْ تَعْدُوَ قَدْرَكَ قَالَ عُمَرُ ائْذَنْ لِي فَأَضْرِبَ عُنُقَهُ قَالَ دَعْهُ إِنْ يَكُنْ هُوَ فَلاَ تُطِيقُهُ وَإِنْ لَمْ يَكُنْ هُوَ فَلاَ خَيْرَ لَكَ فِي قَتْلِهِ