Justin Bieber -- My Mom Gives Me Drugs

Justin Bieber just ratted on one of his drug suppliers to police -- telling cops, HIS MOM is the one who gave him the prescription medication he took shortly before his DUI arrest.

Law enforcement sources tell TMZ, cops grilled Bieber on what substances he had ingested before his arrest ... and the singer admitted he was under the influence of Rx meds, alcohol, and weed.

When asked what kind of Rx meds, we're told Bieber said anti-anxiety (such as Xanax) but he wasn't sure exactly what kind ... because he just takes whatever his mom Pattie gives him.

It's unclear if he has his own prescriptions for the drugs -- and his mom just handles them -- or if he's using his mom's prescriptions ... which would be illegal. If Bieber does have a valid prescription , the presence of the substance in his system probably won't be held against him.

As we reported, when police raided Bieber's house following the infamous egg attack, they found Xanax pills in the home ... so it all syncs up.

As we reported, Bieber also allegedly had a blood alcohol level of .04 ... and that will definitely come back to bite him in the ass.

We've reached out to Bieber's mom ... so far, no comment.