Chief Keef -- 'See Y'all Later' ... I'm Going Back to Jail

8:50 AM PT -- Law enforcement sources tell us Keef has failed 3 random drug tests ... and each time tested positive for marijuana. We're told Keef was sentenced yesterday to a drug rehab facility ... but he's currently being held in the Cook County jail until he can be transferred to the rehab center.

Chief Keef is headed back to jail ... and proudly blasted the news out to the world after a judge slapped him with a 90 day sentence.

Keef, aka Keith Cozart, tweeted ... "Judge Putting me Back in jail See y'all later."

A Chicago judge sent Keef to a jail rehab facility because he reportedly failed a random drug test.

As we previously reported ... the 18-year-old rapper gets tested as part of his probation for a speeding ticket -- cops clocked him doing 110 mph in a 55, back in May.