Hasan on Tazkiyyah: Allah commands you to have a pure heart

Khalid ibn Abi Bakr reported: Al-Hasan al-Basri, may Allah have mercy on him, said, “Cure your heart, for Allah Almighty requires the servants to have sound hearts.”

Source: al-Tawāḍu’ wal-Khumūl 240

عَنْ خَالِدِ بْنِ أَبِي بَكْرٍعَنِ الْحَسَنِ البصري رحمه الله قَالَ دَاوِ قَلْبَكَ فَإِنَّ حَاجَةَ اللَّهِ عَزَّ وَجَلَّ إِلَى الْعِبَادِ صَلَاحُ قُلُوبِهِمْ

240 التواضع والخمول لابن أبي الدنيا