Hawzah News Agency – Imam al-Sadiq (AS), with respect to Allah's verse in the Qur'an: "except him who comes to Allah with a sound heart" said,

القَلبُ السَّليمُ الذي يَلقَى‏ رَبَّهُ، وليسَ فيهِ أحَدٌ سِواهُ، وكُلُّ قَلبٍ فيهِ شِركٌ أو شَكٌّ فهُو ساقِط.

'A sound heart that meets its Lord having nothing other than Him in it. Every heart that has polytheism or doubt is a failed heart.'

Source: al-Kafi, v. 2, p. 16, no. 5