Ibn Abdul-Haadi, Shaikhul-Islaam Ibn Taymiyyah's student wrote:
"And it so happened that a Shaikh from Aleppo came to Damascus, saying, "I have heard that in this city there is a boy who goes by the name of Ahmad the son of Taymiyyah and that he is extremely quick in memorising. I have come perchance I may see him."
So a tailor said to him, "This is his route to school and he has not yet come, so sit with us. Anytime now he will pass us on his way to the school."
So the Aleppan Shaikh sat down for a short while. Then two young boys walked by, so the tailor said to the Shaikh, "The young boy carrying that large tablet - he is Ahmad ibn Taymiyyah."
So the Shaikh called him and he (Ibn Taymiyyah) came to him. Then the Shaikh took the tablet and looked at what was written therein. Then he said, "O my son! Wipe this out so that I can dictate something to you which you can write."
So Ibn Taymiyyah did so. Then the Shaikh dictated eleven or thirteen sayings of the Prophet (sallallaahu alaihi wa sallaam) to him. Then the Shaikh said to him, "Read this."
So Ibn Taymiyyah looked over it once after he had written it and then gave the tablet back to the Shaikh. So he said, "Read it to me."
So he read it back to him in the best way possible. Then the Shaikh said to him, "O my son! Wipe this out."
So he wiped it out and the Shaikh dictated a number of chains of narrations that he had chosen and said, "Read this."
Ibn Taymiyyah looked at it and he did as he had done the first time. So the Shaikh stood up, saying, "If this young boy lives long he will have a great standing and rank - since the like of this has not been seen before."
Ibn Naasirud-Deen narrated the saying of Abul-Muzaffar As-Sarmari, "And from the amazing matters regarding memorisation in our time is the Shaikh of Islaam Abul-Abbaas Ahmad bin Abdul-Haleem bin Taymiyyah. Since he would come across a book and would look over it once only after which it would be inscribed in his mind. He would be able to discuss it thereafter, narrating from it in his written works with its precise wording and meaning.
And from the most amazing of what I have heard about him is what some of his companions narrated to me. They said that in the start of his affair when he was a young boy his father wanted to take his sons for an outing to a garden. So he said to Ibn Taymiyyah, "O Ahmad! Go out with your brothers and relax."
So he exscused himself from that however his father persisted. But the young Ibn Taymiyyah strongly declined, saying, "I would like you to excuse me from going out."
So his father let him stay and left with his other sons. They spent their day in the garden and returned at the end of the day whereupon his father said to him, "O Ahmad! You deserted your brothers today! And you annoyed them because of your absence from them. So what is this?"
So he replied, O father! Today I memorised this book."
Alluding to a book with him. So his father replied, "You memorised it?!"
In denial, shocked and amazed at what his son had just said. So he said to him, "Read it to me."
So he did. He had indeed memorised the entire book. Then his father held him and kissed him between his eyes and said, "O my son! Do not tell anyone about what you have done."
Fearful lest the the Evil Eye should fall upon him."
Taken from Ibn Abdul-Haadi's Al-'Uqood Al-Durriyah min Manaaqib Shaikhil-Islaam Ibn Taymiyyah, p. 7
Translated by Ahmed Abu Turaab