Mishkat al-Masabih 2986
Kharija b. as-Salt quoted his paternal uncle as saying:
We came to a clan of the Arabs after leaving God's Messenger and they said, “We have been told that you have brought what is good from this man. Have you any medicine, or a charm, for we have a lunatic in chains?" When we replied that we had, they brought a lunatic in chains and I recited Fatihat al-Kitab over him three days morning and evening, collecting my saliva and then spitting it out, and he seemed as if he were set free from a bond. They gave me some payment, but I refused to accept it till I had asked the Prophet. When I did so he said, “Accept it*, for by my life, some accept it for a worthless charm, but you have done so for a genuine one." 

Ahmad and Abu Dawud transmitted it.

* Literally "eat”.
عَنْ خَارِجَةَ بْنِ الصَّلْتِ عَنْ عَمِّهِ قَالَ: أَقْبَلْنَا مِنْ عِنْدَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَأَتَيْنَا عَلَى حَيٍّ مِنَ الْعَرَبِ فَقَالُوا: إِنَّا أُنْبِئْنَا أَنَّكُمْ قَدْ جِئْتُمْ مِنْ عِنْدِ هَذَا الرَّجُلِ بِخَيْرٍ فَهَلْ عِنْدَكُمْ مِنْ دَوَاءٍ أَوْ رُقْيَةٍ؟ فَإِنَّ عِنْدَنَا مَعْتُوهًا فِي الْقُيُود فَقُلْنَا: نعم فجاؤوا بِمَعْتُوهٍ فِي الْقُيُودِ فَقَرَأْتُ عَلَيْهِ بِفَاتِحَةِ الْكِتَابِ ثَلَاثَةَ أَيَّامٍ غُدْوَةً وَعَشِيَّةً أَجْمَعُ بُزَاقِي ثُمَّ أَتْفُلُ قَالَ: فَكَأَنَّمَا أُنْشِطَ مِنْ عِقَالٍ فَأَعْطَوْنِي جُعْلًا فَقُلْتُ: لَا حَتَّى أَسْأَلَ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَقَالَ: «كُلْ فَلَعَمْرِي لَمَنْ أَكَلَ بِرُقْيَةِ بَاطِلٍ لَقَدْ أَكَلْتَ بِرُقْيَةِ حَقٍّ» . رَوَاهُ أَحْمد وَأَبُو دَاوُد