Tomori's loan deal with Milan was made permanent on Thursday.
Abraham's said on his Instagram post: “From the ground up, we grew together, we grinded together, even when they doubted us.
“We've come so far! & this is just the start. Good luck in this new challenge. Going to miss you brother."
Abraham, 23, could follow Tomori out of Stamford Bridge this summer after rumours surfaced suggesting he would be included in a player plus cash deal for Borussia Dortmund's Erling Haaland.