How authentic is the hadith about the repentance of the gravedigger? It is found on this site, I think it is a Shia site, but maybe the hadith is authentic? Here is the hadith: http://www.imamreza.net/eng/imamreza.php?id=12599

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 is His slave and Messenger.

We have not come across this story in any of the reliable books that we have at hand, but it is mentioned in some books such as the book entitled Tanbeeh Al-Ghaafileen authored by As-Samarqandi.

In Siyar A’laam An-Nubalaa’, Ath-Thahabi said about the above book, “Abu Al-Layth, the knowledgeable Imaam and scholar of Hadeeth Abul-Layth Nasr ibn Muhammad ibn Ibraaheem As-Samarqandi from the Hanafi School, the author of the book Tanbeeh Al-Ghaafileen… which was known to contain fabricated ahaadeeth.”

Ath-Thahabi also said in his book Taareekh Al-Islam, in the biography of As-Samarqandi, “The book Tanbeeh Al-Ghaafileen includes many fabricated ahaadeeth.”

Ath-Tha’labi also reported this story in his Tafseer (interpretation of the Quran). Shaykh Ibn Taymiyyah said about the book of Ath-Tha’labi, “The Tafseer of Ath-Tha’labi contains what is true and what is false; he is a night wood-cutter (i.e. he includes everything without consideration for authenticity).”

Some scholars said that in this narration cited by Ath-Tha’labi, there is a discontinuity (in the chain of narration) between Az-Zuhri and ‘Umar .

Allaah knows best.