Muwatta Malik, Book 56, Hadith 1831 (In-Book) ⓘ
Malik related to me that he heard that Abdullah ibn Masud used to say, "The slave continues to lie and a black spot grows in his heart until all his heart becomes black. Then he is written, in Allah's sight, among the liars."
وَحَدَّثَنِي مَالِكٌ، أَنَّهُ بَلَغَهُ أَنَّ عَبْدَ اللَّهِ بْنَ مَسْعُودٍ، كَانَ يَقُولُ لاَ يَزَالُ الْعَبْدُ يَكْذِبُ وَتُنْكَتُ فِي قَلْبِهِ نُكْتَةٌ سَوْدَاءُ حَتَّى يَسْوَدَّ قَلْبُهُ كُلُّهُ فَيُكْتَبَ عِنْدَ اللَّهِ مِنَ الْكَاذِبِينَ .
also referenced under: Book 56, Hadith 18 (USC-MSA)