Source: Tafsir al-Ahlam al-Kabir (Ibn Sirin)
Disbelief in a dream indicates poverty. Whoever sees himself disbelieve after having faith will become poor. It may also indicate misguidance and distance from the truth.
Source: Ta'tir al-Anam fi Ta'bir al-Manam (Al-Nabulsi)
Disbelief indicates tyranny and arrogance. Whoever sees himself disbelieving is committing a great sin.
Source: Al-Isharat fi Ilm al-Ibarat (Ibn Shahin)
Disbelief is interpreted as misguidance. Whoever sees himself calling people to disbelief is calling them to heresy.