One strike fills the whole grave..

Abdullah ibn Mas’ood, (Radhi Allahu Anhu), related that the Prophet Muhammad (Salla’Allaahu Alayhi wa Salam), said:

“One of Allah’s slaves was ordered to be beaten in his grave one hundred times. He never ceased asking and supplicating until it became one strike, so he was struck once and his grave became filled with fire. When he rose up and regained consciousness he said:

‘What did you strike me for?’ They said:

‘You prayed a prayer without purity and you passed someone that was oppressed without helping him’”

[Reported by at-Tahawi in “Mushkil al-Aathar” and graded Hasan (good) by Shaykh al-Albaani in “As-Silsilah as-Sahihah”, number: 2774, and he said: “All of the narrators are trustworthy.”]