Abū ʿUmar Al-Shaybānī reports:

I would sit with Ibn Mas’ūd a whole year without him saying “Allah’s Messenger – Allah’s peace and blessings be upon him – said…” When he did actually say “Allah’s Messenger – Allah’s peace and blessings be upon him – said…” he would tremble and add, “…or something similar to this” or “words to that effect.”

Al-Dhahabī, Tadhkirah Al-Huffādh Vol.1 p15.

Imām Al-Dhahabī said:

[Ibn Mas’ūd] was one of those who were very careful and conscientious in the delivery of narrations and he was very strict in reporting; he used to reprimand his students for any negligence in precisely narrating the wordings of narrations.