How about 1 Corinthians 15:3-8? Not an “external” reference (in the NT obviously), but it is a historical letter, a valid document whose content/authorship are not in dispute.
"“For I handed on to you as of first importance what I in turn had received: that Christ died for our sins in accordance with the scriptures, 4 and that he was buried, and that he was raised on the third day in accordance with the scriptures, 5 and that he appeared to Cephas, then to the twelve. 6 Then he appeared to more than five hundred brothers and sisters[c] at one time, most of whom are still alive, though some have died.[d] 7 Then he appeared to James, then to all the apostles. 8 Last of all, as to one untimely born, he appeared also to me.”
Most scholars date the creedal statement here to within 3-5 year of Jesus’ death.