Qalb Saliim

But only one who comes to Allah with a qalbin saliim [sound heart]."

[Qur'aan 26:89]

al-Tabari notes in his Tafsir

والذي عني به من سلامة القلب في هذا الموضع: هو سلامة القلب من الشكّ في توحيد الله, والبعث بعد الممات.

The soundness of the heart being referred to in this place is its being sound from doubt in regards to the Tawhiid of Allaah and the resurrection after death.