Assalaamu alaykum wa rahmatullaah. Is it confirmed that Khalid bin Yazid heard from Abi Hilaal Sa‘eed bin Abi Hilaal before losing his memory? May Allah reward you.
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The correct view is that it was not authentically established that Saʻeed ibn Abi Hilaal lost his memory to begin with. None of the Hadith scholars who composed books on the narrators who suffered loss of memory (Ikhtilaat) mentioned him. The only Hadith scholar who mentioned this was Ibn Hajar in At-Taqreeb. He said, "He was a trustworthy reporter; I did not find anyone who judged him to be a weak reporter before Ibn Hazm, except that As-Saaji narrated from (Imam) Ahmad that he (Saʻeed ibn Hilaal) lost his memory." [At-Taqreeb]
From the contemporary scholars, Al-Albaani held the same view.
The exact statement of Imam Ahmad cited in Tahtheeb At-Tahtheeb reads, "As-Saaji said, 'He is trustworthy. Ahmad used to say, ‘I do not know what he confuses in his narrations.’'"
This statement does not indicate Ikhtilaat in its terminological sense, which is used to describe a trustworthy reporter endowed with a strong memory who becomes confused in his narration and whose memory fails, as underlined by Dr. Saʻd Al-Hameed in his commentary and revision of the book "Tafseer Saʻeed ibn Mansoor (3/833)" and Dr. Ibraaheem Al-Laahim in his book "Al-Jarh wa At-Taʻdeel (156)."
There is an Arabic research paper which was published online and is entitled "Tarjamat Saʻeed ibn Abi Hilaal, Haafith Misr wa Radd Daʻwa Al-Ikhtilaat" (Biography of Sa'eed ibn Abi Hilaal, the Haafith of Egypt, and Refutation of the claim that he lost his memory). You can refer to it for more information in this regard (if you can read Arabic).
Allah knows best.
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