Hawzah News Agency – The merciful prophet of Islam, peace be upon him and his household says:

إنّ النِّفاقَ يَبدو لُمظَةً سَوداءَ، فكُلّما ازدادَ النِّفاقُ عِظَماً ازدادَ ذلكَ السَّوادُ، فإذا استَكمَلَ النِّفاقُ اسوَدَّ القَلبُ.

'Hypocrisy appears as a black dot [on the heart]. Whenever hypocrisy increases in magnitude that black dot increases, and when hypocrisy becomes complete the heart becomes black.'

Source: Kanz al-Ummal, no. 1734