Ibn Taymiyyah wrote the entire book ‘as-Sarim al-Maslul’ from memory!
“And from the strangest of things in regards to this is that during his first trial in Egypt, he was taken and jailed, such that he was prevented from having access to his books. During this time, he authored many books – small and large – and mentioned in them what was of ahadith, narrations, statements of the Companions, names of the scholars of Hadith, authors and their works – and he attributed each of these to their proper sources, specifically by name. He also mentioned the names of the books in which each narration was found, as well as where in the books to find them. All of this was purely from his memory, as at the time, he did not have a single book with him to use as a reference. These books were then published and looked over, and – praise be to Allah – not a single mistake was found in any of them, nor did anything need to be changed in them.
And from these books is ‘as-Sarim al-Maslul ‘ala Shatim ar-Rasul,’ and this is from the virtue that Allah – the Exalted – reserved especially for him.”
And to give you an idea of the vastness of the book, the publisher’s introduction goes on to mention that it contains over 250 ahadith, 100 athar, the mention of over 600 famous personalities throughout the history of Ahl as-Sunnah wal-Jama’ah, collected information from over 40 references (and these are just the ones he mentioned by name) – all from memory, and this entire book was written in response to one single incident in which Ibn Taymiyyah heard a Christian insulting the Prophet!