Abu Hurayrah narrated 5,374 Hadith.
It is well documented that he used to write Hadith during the time of the Messenger of Allah {peace and blessings of Allah be upon him, his family and companions}.
He says:
Once I asked the Messenger of Allah:
“Should I write down everything which I have heard from you?”
He said, “Yes”. I asked, both what you say happily and angrily. He said, “Yes”, as I don’t say anything except truth in all these states.”
{Sunan ad-Darimi, also related by Hafiz ibn Abd al-Barr in his at-Tamhid & Istidhkar}
