I heard the Messenger of Allah ﷺ say: He then mentioned a tradition like it. This version adds: When they propound that, say: "Say Allah is one. Allah is He to Whom men repair. He has not begotten and He has not been begotten, and no one is equal to Him." Then one should spit three times on his left side and seek refuge in Allah from Satan.  

سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ فَذَكَرَ نَحْوَهُ قَالَ فَإِذَا قَالُوا ذَلِكَ فَقُولُوا { اللَّهُ أَحَدٌ * اللَّهُ الصَّمَدُ * لَمْ يَلِدْ وَلَمْ يُولَدْ * وَلَمْ يَكُنْ لَهُ كُفُوًا أَحَدٌ } ثُمَّ لْيَتْفُلْ عَنْ يَسَارِهِ ثَلاَثًا وَلْيَسْتَعِذْ مِنَ الشَّيْطَانِ  

[Machine] I heard the Messenger of Allah ﷺ saying, "People will soon cause confusion among themselves until someone among them will say, 'Who created Allah?' When they say that, respond by saying, 'Allah is One, Allah is the Eternal. He neither begets nor was He begotten, and there is no one comparable to Him.' Then, let them spit to their left three times and seek refuge in Allah from Satan." Umar said, "Then let them spit to their left three times and seek refuge in Allah from Satan."  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ يُوشِكُ النَّاسُ أَنْ يَتَسَاءَلُوا بَيْنَهُمْ حَتَّى يَقُولَ قَائِلُهُمْ هَذَا اللهُ خَلَقَ الْخَلْقَ فَمَنْ خَلَقَ اللهُ؟ فَإِذَا قَالُوا ذَلِكَ فَقُولُوا {اللهُ أَحَدٌ اللهُ الصَّمَدُ لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كُفُوًا} [الإخلاص 2] أَحَدٌ ثُمَّ لِيَتْفُلْ عَنْ يَسَارِهِ ثَلَاثًا وَلْيَسْتَعِذْ بِاللهِ مِنَ الشَّيْطَانِ وَقَالَ عَمْرٌو ثُمَّ لِيَتْفُلْ عَنْ يَسَارِهِ ثَلَاثًا وَلْيَتَعَوَّذْ مِنَ الشَّيْطَانِ