Sources close to the adoption tell TMZ Khalid got put through the ringer Saturday -- with a 4-hour long interview and vetting process at L.A.'s Bark n' Bitches.

We're told BNB had the R&B singer fill out extensive paperwork and provide references to make sure he was up to the task -- considering he's only 19 and constantly touring the country. Lucky for Khalid, we're told his manager offered a glowing reference on the spot, and he was approved.

In the end, he walked out with a 9-week-old pit bull mix named Roxy. We're told his total cost was several hundred bucks ... including treats, tags, toys and a bed.