This is a well known incident agreed upon by both shia and sunni sides.

QUOTE
The Most Noble Messenger, peace and blessings be upon him and his family, appointed Ali, peace be upon him, to guard the city on his behalf, and he ordered that he should remain in Madinah until he returned, administering the affairs of the Muslims. When the Hypocrites realized that their treacherous plans had been divulged, they began spreading idle rumors in the hope of weakening Ali's position. They hinted that the Prophet, peace and blessings be upon him and his family, was angry with Ali, peace be upon him, and that it was for this reason that he had not been permitted to accompany him on a major military expedition.

Ali, peace be upon him, was greatly troubled and saddened by the circulation of these rumors, and he hastened to the presence of the Prophet, peace and blessings be upon him and his family, who had already left Madinah.

http://home.swipnet.se/islam/books/imamate/lesson5.htm


Now read this

Narrated Ibn 'Abbas:
When the time of the death of the Prophet approached while there were some men in the house, and among them was 'Umar bin Al-Khatttab, the Prophet said, "Come near let me write for you a writing after which you will never go astray." 'Umar said, "The Prophet is seriously ill, and you have the Quran, so Allah's Book is sufficient for us." The people in the house differed and disputed. Some of them said, "Come near so that Allah's Apostle may write for you a writing after which you will not go astray," while some of them said what 'Umar said. When they made much noise and differed greatly before the Prophet, he said to them, "Go away and leave me." Ibn 'Abbas used to say, "It was a great disaster that their difference and noise prevented Allah's Apostle from writing that writing for them.

Now shias criticize Umar (may Allah be pleased with him) but not Ali (may Allah be pleased with him). Why ? When their belief is that Umar (may Allah be pleased with him) is to be condemned for disobeying to the Prophet (peace be upon him) whatever the reason, than by applying the same logic, what would they like to assume about Ali (may Allah be pleased with him) ?

It is clear that both did this for what they considered better or if just accusation is the rule of the day, than answer me, didn't Ali (may Allah be pleased with him)

1. Disobeyed the Prophet (peace be upon him) (as per shia's logic , Umar (may Allah be pleased with him) disobeyed whatever the case , apply the same logic and same is the result)

2. Left the city at the mercy of hypocrites 3. without verifying the news and thus going against the Quranic verse [049:006] O ye who believe! If a wicked person comes to you with any news, ascertain the truth, lest ye harm people unwittingly, and afterwards become full of repentance for what ye have done. 4. Going against the shia belief that imams have the knowledge of the unseen, either Ali (may Allah be pleased with him) is not an imam then, and either he didn't have the knowledge of the unseen and thus going against the very basic beliefs of shia people. Because if he had the knowledge of the unseen , how come he didn't know the truth of the matter and had to hasten towards the Prophet (may Allah be pleased with him) to know the reality.


What i believe is that what Ali and Umar (may Allah be pleased with them) did in these circumstances was what they thought the best , and the lie of the imams having the knowledge of the unseen as shias' think should be very clear by now

[007:188] Say: "I have no power over any good or harm to myself except as God willeth. If I had knowledge of the unseen, I should have multiplied all good, and no evil should have touched me: I am but a warner, and a bringer of glad tidings to those who have faith."