How does Allah nullify deeds, and could this include the Companions?

The nullification of the reward for good deeds occurs through associating partners with Allah the Almighty (Shirk (definition)) and apostasy from Islam. The proof for this is Allah the Almighty's saying: "And whoever denies the faith - his work has become worthless, and he, in the Hereafter, will be among the losers" [Al-Ma'idah: 5], and His saying: "But if they had associated others with Allah, then [all] that they did would have become worthless for them" [Al-An'am: 88], and His saying: "And it was already revealed to you and to those before you that if you should associate [anything] with Allah, your work would surely become worthless, and you would surely be among the losers" [Az-Zumar: 65]. The nullification of deeds due to apostasy is conditional upon dying in disbelief, as stated in His saying: "And whoever of you reverts from his religion and dies while he is a disbeliever - for those, their deeds have become worthless in this world and the Hereafter" [Al-Baqarah: 217].

As for sins other than Shirk, they contend with good deeds, and the dominant one prevails. The righteous predecessors feared the nullification of the reward for deeds due to disobeying Allah's commands, as happened to Umar ibn Al-Khattab and Thabit ibn Qays, may Allah be pleased with them both, after the revelation of Allah the Almighty's saying: "O you who have believed, do not raise your voices above the voice of the Prophet or be loud to him in speech like the loudness of some of you to others, lest your deeds become worthless while you perceive not" [Al-Hujurat: 2]. Al-Bukhari pointed to the chapter on the believer's fear that his deeds might become worthless without him realizing it. It was narrated from Ibrahim At-Taimi, Ibn Abi Mulaikah, and Al-Hasan (definition) Al-Basri their fear of hypocrisy and the nullification of deeds.