Mishkat al-Masabih 75
Abu Huraira reported God’s messenger as saying, "Men will continue to question one another till this is propounded:
God created all things, but who created God? When they propound that, say, ‘God is one. God is He to whom men repair. He has not begotten and He has not been begotten, and no one is equal to Him.” 1 Then one should spit three times on his left side and seek refuge in God from the accursed devil.” 

Abu Dawud transmitted it. We shall mention the tradition of ‘Amr b. al-Ahwas in the chapter on the sermon on the day of sacrifice, if God will. 1 Book XI; Ch. xi
عَنْ أَبِي هُرَيْرَةَ قَالَ سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسلم يَقُول: 
" لَا يَزَالُ النَّاسُ يَتَسَاءَلُونَ حَتَّى يُقَالَ: هَذَا خَلَقَ اللَّهُ الْخَلْقَ فَمَنْ خَلَقَ اللَّهَ؟ فَإِذَا قَالُوا ذَلِك فَقولُوا الله أحد الله الصَّمد لَمْ يَلِدْ وَلَمْ يُولَدْ وَلَمْ يَكُنْ لَهُ كفوا أحد ثمَّ ليتفل عَن يسَاره ثَلَاثًا وليستعذ من الشَّيْطَان ". رَوَاهُ أَبُو دَاوُد