What are the sins that the Companions feared the consequences of, even though the Messenger of Allah, peace and blessings be upon him, gave glad tidings to some of them of Paradise and salvation from Hell, and can a Muslim reach their level by adhering to Allah's commands?

The Companions feared falling into hypocrisy and sins that would invalidate good deeds. An example of this is Thabit ibn Qays's fear that the verse: {O you who have believed, do not raise your voices above the voice of the Prophet} was revealed concerning him, so he sat sorrowfully in his house. Ibn Abi Mulayka mentioned that he met thirty Companions, all of whom feared hypocrisy for themselves. Al-Hasan (definition) said: "None fears it except a believer, and none feels secure from it except a hypocrite." The more a servant knows Allah and His religion, the more fearful and pious he becomes.