Muwatta Malik 1828

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 ‏.‏

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

Reference

Muwatta Malik 1828

In-book

Book 56, Hadith 18