Abu Sa'd al-Sam'ani: He was the most knowledgeable hadith scholar of his time, and he was one of the best people in terms of hearing and knowledge of hadith principles.
Abu Abd Allah al-Hakim al-Naysaburi: He was the most knowledgeable hadith scholar of his time, and one of the most knowledgeable people in hadith principles.
Al-Dhahabi: The Imam, the حافظ, the knowledgeable scholar, and once: the trustworthy, well-traveled hadith scholar
Khalil bin Ayyub al-Safadi: He was the most knowledgeable hadith scholar of his time, and one of the most knowledgeable people in hadith principles.