In Paul's letters, Peter is called the first witness to the
Resurrection.
1 Corinthians 15:3-5
... Christ died ... was buried ... was raised on the
third
day ... that he appeared to Kephas, then to the Twelve.
Paul repeatedly call Peter "Kephas," the name given him
by
Christ.
Galatians 1:18
Then after three years I went up to Jerusalem to confer
with Kephas
and remained with him for fifteen days.