Language:
كتاب الأدب
78  
Good Manners and Form (Al-Adab)
(117)
Chapter: The description of something by a man as 'nothing' while he means that it is not true
(117)
باب قَوْلِ الرَّجُلِ لِلشَّىْءِ لَيْسَ بِشَىْءٍ وَهْوَ يَنْوِي أَنَّهُ لَيْسَ بِحَقٍّ‏‏
Sahih al-Bukhari 6213

Narrated `Aisha:

Some people asked Allah's Messenger (ﷺ) about the fore-tellers. Allah's Messenger (ﷺ) said to them, "They are nothing (i.e., liars)." The people said, 'O Allah's Messenger (ﷺ) ! Sometimes they tell something which comes out to be true." Allah's Messenger (ﷺ) said, "That word which comes to be true is what a jinx snatches away by stealing and then pours it in the ear of his fore-teller with a sound similar to the cackle of a hen, and then they add to it one-hundred lies."

حَدَّثَنَا مُحَمَّدُ بْنُ سَلاَمٍ، أَخْبَرَنَا مَخْلَدُ بْنُ يَزِيدَ، أَخْبَرَنَا ابْنُ جُرَيْجٍ، قَالَ ابْنُ شِهَابٍ أَخْبَرَنِي يَحْيَى بْنُ عُرْوَةَ، أَنَّهُ سَمِعَ عُرْوَةَ، يَقُولُ قَالَتْ عَائِشَةُ سَأَلَ أُنَاسٌ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنِ الْكُهَّانِ فَقَالَ لَهُمْ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لَيْسُوا بِشَىْءٍ ‏"‏‏.‏ قَالُوا يَا رَسُولَ اللَّهِ فَإِنَّهُمْ يُحَدِّثُونَ أَحْيَانًا بِالشَّىْءِ يَكُونُ حَقًّا‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ تِلْكَ الْكَلِمَةُ مِنَ الْحَقِّ يَخْطَفُهَا الْجِنِّيُّ، فَيَقُرُّهَا فِي أُذُنِ وَلِيِّهِ قَرَّ الدَّجَاجَةِ، فَيَخْلِطُونَ فِيهَا أَكْثَرَ مِنْ مِائَةِ كَذْبَةٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 6213
In-book reference : Book 78, Hadith 237
USC-MSA web (English) reference : Vol. 8, Book 73, Hadith 232
  (deprecated numbering scheme)