Will Smith backed up his wife, Jada Pinkett Smith, after she allegedly threatened to kill his former friend who was planning to publish a book with details about the Smith family ... this according to newly legal docs filed by the former friend.

Bilaal Salaam, the former friend suing Jada, dropped the bombshell in newly filed court docs ... claiming the conversation went down on or around October 5, 2021.

Bilaal said he was on the call with Will, Jada, and Will's other friend, Charlie Mack. He claims Jada told him she would have him killed if he continued to move forward with a planned memoir talking about his time with the Smiths.

In his filing, Bilaal says he asked Will if he heard what Jada said ... and claims Will told him, "You heard what my wife said."

He said he told Jada and Will not to contact him again in October 2021. He claims he was called by Duane Martin in March 2022 after Will's infamous Oscars slap ... claiming Duane asked him to help clean up the PR mess ... but Bilaal says he refused.

Bilaal said he took the alleged threats seriously because of Will and Jada's power and wealth.

As TMZ previously reported, Bilaal sued Jada for emotional distress that he claims she caused him.