According to `Abdallaah - his father - al-Fudayl - `Abdallaah: I was told that `Umar bin `Abd al-Azeez wrote to the Syrian army as follows:

"As-salaamu `alaikum wa rahmatullaah. Now then, whoever contemplates death frequently speaks little, while he who knows that death is certain is satisfied with a little. Farewell."

For variant versions of this sermon, see Ibn `Abd al-Hakam, Seerah 43-45, 132-33; Ibn Katheer, Bidaayah, IX, 199; This translation was taken from The History of al-Tabaree, Vol XXIV, by D.S.Powers/ http://www.islaam.com/Article.aspx?id=83