MediaTakeOut.com just got word from an extremely credible source that something popped off last night between Justin Timberlake and R&B singer Rihanna.

According to this source, Rihanna snuck out of the Palms hotel after-party with Justin and the two weren't seen again for the rest of the night. Go figure.

The insider told MediaTakeOut.com, "Justin and Rihanna were hanging out and [talking] for a while. Then they both got up all nonchalant and left together … It was crazy because Jessica [Biel] was there and she was looking all over for [Justin] … Poor girl. By the way Justin and [Rihanna] were looking at each other, you know exactly what they were going off to do."

Rihanna's back to her man-stealing ways….