Umar didn't know the laws of Tayyammum.

http://www.usc.edu/dept/MSA/fundamentals/hadithsunnah/muslim/003.smt.html#003.0718

Book 003, Number 0716:
Shaqiq reported: I was sitting in the company of Abdullah and Abu Musa when Abu Musa said: 0 'Abd al-Rahman (kunya of 'Abdullah b. Mas'ud), what would you like a man to do about the prayer if he experiences a seminal emission or has sexual intercourse but does not find water for a month? 'Abdullah said: He should not perform tayammum even if he does not find water for a month. 'Abdullah said: Then what about the verse in Sura Ma'ida:" If you do not find water, betake yourself to clean dust"? 'Abdullah said: If they were granted concession on the basis of this verse, there is a possibility that they would perform tayammum with dust on finding water very cold for themselves. Abu Musa said to Abdullah: You have not heard the words of 'Ammar: The Messenger of Allah (may peace be upon him) sent me on an errand and I had a seminal emission, but could find no water, and rolled myself in dust just as a beast rolls itself. I came to the Messenger of Allah (may peace be upon him) then and made a mention of that to him and he (the Holy Prophet) said: It would have been enough for you to do thus. Then he struck the ground with his hands once and wiped his right hand with the help of his left hand and the exterior of his palms and his face. 'Abdullah said: Didn't you see that Umar was not fully satisfied with the words of 'Ammar only?