One day, upon passing by the monastery of a monk, ‘Umar (ra) called out, “O monk!” at which point the monk appeared [above his fence or his outer wall].” ‘Umar (ra) fixed his gaze upon the monk, and as he stared at him, he began to cry. Someone said, “O Leader of the Believers, why is he making you cry?” ‘Umar (ra) said, “[By looking at him] I remembered the saying of Allah – the
Possessor of Might and Majesty – in His Book:”
عَامِلَةٌ۬ نَّاصِبَةٌ۬ (٣) تَصۡلَىٰ نَارًا حَامِيَةً۬ (٤)
“Labouring (hard in the worldly life by worshipping others besides Allâh), weary (in the Hereafter with humility and disgrace). They will enter in the hot blazing Fire.” (Qur’an 88:3-4)
‘Umar (ra) went on to say, “That is what made me cry.”
[The Biography of Umar Ibn Al-Khattaab by Dr. Ali Muhammad Sallabi, Vol.1, p. 106-107.]
