Uh, it was said by Kabuto himself
''Hiruzen was one of the most powerful shinobi in history, so much so that he was admired as the God of Shinobi (忍の神, Shinobi no Kami) in his prime and was said to be the strongest Hokage that Konoha ever had.[1][20][21] Even in his old age, he also said to be the strongest of all the five Kage at the time of his death.[22] Enma commented on his battle with Orochimaru, saying that it was "miserable" compared to his prime, even though he had clearly been able to hold his own against Orochimaru and two former Hokage without losing his composure. Even in his old age, his chakra was noted for being very strong, causing a dense effect on the atmosphere when released;[23] furthermore, he was known for having immense reserves of chakra in his youth.[24] He was also the longest living and longest reigning Hokage. Hiruzen also had keen senses, being able to track down the former Hokage under Hashirama Senju's Bringer-of-Darkness Technique.[25]''
''Hiruzen was also called The Professor (プロフェッサー, Purofessā), due to his mastery of all forms of shinobi combat, and his vast knowledge of techniques, purportedly knowing all the techniques within Konoha.''
This is from the manga by the way, so yeah. We haven't got to see Hiruzen in his prime yet, and I don't know if we will