Abû ‘Umar Al-Shaybânî reports:
I would sit with Ibn Mas’ûd 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-Dhahabî, Tadhkirah Al-Huffâdh Vol.1 p15.
Imâm Al-Dhahabî said:
[Ibn Mas’ûd] 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.