Is there are an address (khutbah) ascribed to Ali (rta ) about Umar's (rta) death?
We read in Nahj al-Balagha (a book compiled by Sayyed Radhi, a Shia scholar, containing sermons, letters and short statements attributed to Ali - ra) the following:
"May Allah reward 'such and such man' who straightened the curve, cured the disease, abandoned mischief and established the Sunnah. He departed (from this world) with untarnished clothes and little shortcomings. He achieved good (of this world) and remained safe from its evils. He offered Allah's obedience and feared Him as He deserved. He went away and left the people in dividing ways wherein the misled cannot obtain guidance and the guided cannot attain certainty."
Ibn Abil-Hadid a Mutazeli scholar who has written the most famous commentary of Nahj al-Balagha writes that 'such and such man' ('Fulan' in original Arabic) refers to Umar (ra) as he has seen in a Sayyed Radhi's hand written manuscript of Nahj al-Balagha that the word Umar was written beneath 'Fulan' (Sharh Nahj al-balaghah, 14: 3-4).
This is supported by the fact that Tabari too reports a similar saying of Ali (ra) about Umar (ra) on the occasion of his death (Tabari, 4:218)