It is reported that Al-Hasan Al-Basrî – Allâh have mercy on him – said:

The believer does the best deeds yet is most fearful [that his deeds will not be accepted]. If he were to spend a mountain of wealth [in charity], he would not feel sure [of the reward] until he sees it. The more righteous and pious he becomes, the more he fears. But the hypocrite (munâfiq) says, ‘There are so many people, I will be forgiven, no problem.’ So he does wrong and evil deeds, yet holds foolish wishes about Allâh.

Al-Dhahabî, Siyar A’lâm Al-Nubalâ` 4:586.