All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
This account is part of a report that was narrated on the authority of Suhayb Ar-Roomi and it reads, "It was mentioned that he was dug out during the time of ʻUmar ibn Al-Khattaab and that his finger was on his temple, just as he had placed it when he was killed." [At-Tirmithi] Al-Albaani declared the report saheeh (sound).
The statement about which you are asking was cited by Al-Mubaarakfoori and he commented on it saying:
"Ibn Is-haaq said, 'I was told by ʻAbdullaah ibn Abi Bakr ibn Muhammad ibn ʻAmr ibn Hazm that he was told that in the days of ʻUmar ibn Al-Khattaab, a man from Najraan dug up one of the ruins of Najraan intending to make use of the land when he came upon ʻAbdullaah ibn At-Taamir buried therein; he was in a sitting posture with his hand covering a wound in his head and holding firmly to it. When his hand was removed, the blood began to flow; when they let go of his hand, it returned to its place and the flow of blood ceased. On his finger was a ring with the inscription 'Allaah is my Lord'. A message was sent to ʻUmar about it, and he replied, ‘Leave him as he is and bury him as he was buried,’ and his orders were carried out.'” [Sharh Tuhfat Al-Ahwathi]
Allaah knows best.