Is the hadith attributed to Omar Ibn Al-Khattab, "If a thunderbolt were to descend from the the sky, it would not strike one who is seeking forgiveness," authentic?

We do not know whether the statement "frequent seeking of forgiveness (istighfar) averts calamities" was uttered by Umar (may Allah be pleased with him) or any other Companion. The words of Allah the Almighty suffice instead: ﴿And Allah would not punish them while you [Muhammad] are among them, nor would He punish them while they seek forgiveness﴾ [Al-Anfal/33]. There were two safeguards against punishment: one was removed, which was the presence of the Prophet (peace be upon him), and the other remains, which is seeking forgiveness.