Acts 22:20
And when the blood of thy martyr Stephen was shed, I also was standing by, and consenting unto his death, and kept the raiment of them that slew him.
Cross-reference
Acts 22:4 is Paul's own summary of persecuting Christians to death, which includes his role in Stephen's stoning.
Acts 7:58 narrates Stephen's stoning with witnesses laying garments at Saul's feet, directly showing the event Paul summarizes here.
Acts 8:1 states that Saul approved of Stephen's execution, confirming Paul's own confession in Acts 22:20 that he was approving.
Acts 9:13 confirms Paul's reputation as a persecutor of saints, including Stephen, from Ananias's perspective.
Acts 14:19 recounts Paul being stoned—the same fate he approved for Stephen, showing a reversal of roles.
Acts 26:10 has Paul saying he cast his vote against Christians put to death, confirming his involvement in Stephen's case.
In Luke 11:48, Jesus condemns approving of ancestral prophet-killings; Paul did exactly that to Stephen, showing his sin fit Jesus' indictment.
Luke 11:49 says God's messengers will be killed; Stephen was such a messenger, and Paul approved his death.
Romans 1:32 condemns giving approval to sin; Paul's approval of Stephen's murder matches that description, illustrating his former life under sin.