Scholars of the previous generations emphasized the importance of remembering things in memory. As soon as hadith began being collected and passed down, it was extremely important for people to have strong memories in order for the hadith to not be corrupted. Much of the authentication regarding hadith depends on strong memory and reliable writing.
Hisham ibn Abdul Malik, one of the Umayyad khalifas, wanted to test al-Zuhri one day. So he called him over and said his sons wanted to learn some hadith so could he please dictate some hadith for them to write. So al-Zuhri sat and narrated 400 hadith by heart. After a month, Hisham called al-Zuhri back again and said the books they had written had gotten lost. So al-Zuhri said this was no problem and he sat and narrated the same 400 hadith again from memory, while the scribes wrote. They compared the book written then to the one a month ago and not a single letter was missing.
It was said that al-Zuhri used to put cotton in his ear in the marketplace so that he would not be corrupted by what people are saying. This way he could always keep his mind focused on the words of hadith.