Witnesses to Jesus's resurrection

Growing up in a Christian household I've heard many times "there are loads of written testimonies of those who saw Jesus's resurrection and even secular sources mention it"

Even an article i read recently >There are many documents, manuscripts and fragments of the Bible, Old and New Testament, in addition to secular sources that speak of Christ’s resurrection.

>God has ensured that the historical record of Christ’s resurrection would be preserved accurately for us.

But I've never seen anyone actually site these sources, just allude to them. Ive checked and have never seen them just people mention them. So my question is, where are these documents outside of the bible that have the testimonies from eye witnesses? Are therr actually any? Or are people mistaken?