Break it down for me

The ricin cigg. Jesse knew the day the kid was in the hospital Walt was involved. He threatens him and Walt laughs it off and talks him out of it. “I was waiting for his to send someone and it’s you” etc

When the vacuum dude shows, why does Jesse know Walt was involved? Why is that when he knows for sure? I’ve watched the series like 15 times and I don’t know. 🤣