Reference: Siyar A’laam an-Nubalaa – Volume 9, Page 98
Yahya ibn Ma’een said:
That day, Ghundar brought out a sack of books, and said: ‘Do your best to find an error in them.’
So we [strived hard, and] did not find an error in any of them.
And [as for Ghundar], he [was a man who] would fast every other day for the past 50 years.