This is not a Shia belief. We respect the companions of the holy prophet (pbuh) as per our teachings of Imams (as). Imam Sajjad (as)'s dua in al-Sahifa al-Sajjadiya reveals how the true followers of the holy prophet (pbuh) were revered by Imams (as).
Not all the companions adhered to the true teachings of the holy prophet (pbuh) and many had turned away from the teachings of the holy prophet (pbuh) regarding the Imamat and Caliphate of Imam Ali (as). But this doesn't make them apostate. We should respect them with high regard as mentioned in Qur'an 9:100.
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