He would regularly broadcast in Arabic from Berlin, and these transmissions were relayed to other axis radio transmitters for the Arab world to hear.
In March of 1944, speaking on Berlin radio he said:
Arabs! Arise as one and fight for your sacred rights. Kill the Jews wherever you find them. This pleases God, history, and religion. This saves your honor. God is with you.
Some sources say March 1st others say March 4th.
Asia and the Americas (1946)
The Central European Observer (1946) (says "March 1, 1944")
Mufti of Jerusalem; the story of Haj Amin el Husseini (1947) (says "12.30 hours on 1st March, 1944")
Behind the Silken Curtain (1947) (says "on March 4, 1944, when the United States Monitoring Service heard him broadcast")
Phylon: Atlanta University Review of Race and Culture, vol. 9, page 319 (1948) (alternative JSTOR link)(says "March 4, 1944, over Radio Berlin again")
Congressional Record (1948)
According to The Mufti and the Fuehrer: the rise and fall of Haj Amin el-Husseini a more complete quotation is:
No one ever thought that 140,000,000 Americans would become the hands of the Jews....How would the Americans dare to Judaize Palestine while the Arabs are still alive?...The wicked American intentions toward the Arabs are now clear, and there remain no doubts that they are endeavoring to establish a Jewish empire in the Arab world. More than 400,000,000 Arabs [?] oppose this criminal American movement.... Arabs! Rise as one and fight for your sacred rights. Kill the Jews wherever you find them. This pleases God, history, and religion. This saves your honor. God is with you.
More recent sources indicate that the primary source is:
Weekly Review of Foreign Broadcasts, F.C.C. [Federal Communications Commission], No. 118 3/4/44, Near and Middle East,” United States National Archives, College Park, Record Group 165, MID, Regional File, 1922-44, Palestine, entry 77, Box 2719, folder 2930
In other words, the primary source is stored at this location: https://www.archives.gov/dc-metro/college-park
Particularly, see at https://www.archives.gov/research/holocaust/finding-aid/military/rg-165.html#Intelligence the listing of 3335 boxes under:
Formerly Security-Classified Intelligence Reference Publications ("Regional File") Received From U.S. Military Attaches, Military and Civilian Agencies of the United States, Foreign Governments, and Other Sources, 1922-1944 (Entry 77)