User blog comment:Legionaire22/Angron VS Leman/@comment-10514705-20110716135351/@comment-10514705-20110717221634

Yup, Supah's right, The direct quote is:

"At the time of the Horus Heresy it is said that no other Primarch could have bested Angron in single combat save for Horus and perhaps Sanguinius"

And according to the Lexicanum article this was mentioned in Raven's Flight.