It’s true. Here is the statement attributed to Muhammad to which al-Harbi is referring:
“Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: The last hour would not come unless the Muslims will fight against the Jews and the Muslims would kill them until the Jews would hide themselves behind a stone or a tree and a stone or a tree would say: Muslim, or the servant of Allah, there is a Jew behind me; come and kill him; but the tree Gharqad would not say, for it is the tree of the Jews.” (Sahih Muslim 6985)
But as far as the ADL is concerned, if you take note of this, you (not al-Harbi) are a racist bigot.
In a lecture titled “Who Will Liberate Al-Aqsa,” Saudi cleric Sheikh Mamdouh Al-Harbi said that the Muslims’ war is with the Jews and not just with Zionists and that saying otherwise “constitutes a denial of the words of Allah and of the Prophet Muhammad.” The lecture was posted on Al-Harbi’s YouTube channel on July 26.
Sheikh Mamdouh Al-Harbi: “The Prophet Muhammad foretold that we would kill the Jews. He foretold that we would kill the Jews. Let nobody claim [that the Prophet talked] about us killing the Zionists. Anyone who claims that our war is with the Zionists rather than the Jews is mistaken. This constitutes a denial of the words of Allah and of the Prophet Muhammad. Our war is with the Jews, not [just] with the Zionists, even though the Zionists are among the most dangerous kind of Jews. But the war is with the Jews. What Allah said in the Quran, and what the Prophet Muhammad said in the true Sunna… The Prophet did not say: ‘the Zionists.’ He said: ‘the Jews’ – even though the Zionists are among the worst of them. So I say: Our war on Judgement Day will be with the Jews.
“The Prophet Muhammad said: ‘Judgment Day will not come until the Muslims fight the Jews. The Muslims will kill them, and the Jews will hide behind stones and trees.’
“‘The stones and trees will say: “Oh Muslim, oh servant of Allah…”‘ Pay attention now: ‘”Oh Muslim, oh servant of Allah, there is a Jew behind me. Come and kill him.”‘ The Prophet said: ‘Except for the Gharqad tree, which is the tree of the Jews.’ The Gharqad tree is an ugly tree, full of thorns. That tree will not speak because it is with the Jews.”
Sources: MEMRI and Jihad Watch