The argument between Cash App founder Bob Lee and the man accused of stabbing him to death allegedly centered around the suspect's sister ... according to prosecutors.
In court docs, obtained by TMZ, prosecutors say Lee and Tech start-up owner Nima Momeni had a spat before the stabbing ... and a witness says it was about whether Momeni's sister, Khazar, was doing drugs or anything inappropriate.
Prosecutors say the witness claims Lee tried to talk Momeni down and reassure him about Khazar. According to the witness that convo happened at some point in the afternoon or evening of April 3.
From there, investigators have pieced together the events leading up to Bob's death using videos. According to the docs, Bob left his apartment at 12:31 AM on April 4 ... and took an 8-minute walk to Khazar's apartment. Around 2 AM, Nima shows up at Khazar's apartment, and, a few minutes later, he and Bob leave together in Nima's car and drive to a secluded area.
When they get out of the car, Nima allegedly uses a kitchen knife to stab Bob 3 times ... once directly in the heart. Again, prosecutors say they have video of the stabbing. As we've reported, surveillance video -- obtained by Daily Mail -- shows Bob at 2:35 AM, injured and bleeding and desperately looking for help.
The docs say Khazar even texted Lee after the argument, making sure he was ok -- writing "Nima came wayyyyyy down hard on you."
She added, "Thank you for being such a classy man handling it with class Love you Selfish pricks."
As we reported, Momeni was arrested Thursday for the murder of Lee ... and it turns out Momeni has a history of arrests -- including a DUI in 2004 and illegal possession of a switchblade in 2011.
Cops said Thursday they searched Khazar's upscale apartment as part of their investigation -- which was just down the street from a hotel where Lee was staying.