"Saturday ... good evening." Voldemort repeated. His red eyes stared at Snape's black eyes, and his eyes were so sharp that several people next to him quickly looked away, as if they were worried that the ferocious eyes would burn themselves. Snape quietly looked at Voldemort's face. After a while, Voldemort's lipless mouth twisted into an odd smile.

"Come, very good. This information comes from-"

"From the source we discussed." Snape said.

After reading this article, we can know (Deathly Hallows Chapter 32 The Story of the Prince P507)

"You must tell Voldemort the exact date when Harry left his uncle's house during his period," said Dumbledore. "Voldemort thinks you are very well informed. If not, it will arouse his suspicion. However, you must instill the idea of using body double in others-I think that should ensure Harry's safety. Try to use the confusion spell on Mundungus Fletcher. Also, Severus, if you have to take part in the hunt, you must be convincing ... I count on you to continue to gain Voldemort's trust for as long as possible, otherwise, Hogwarts will be at the mercy of Carlo's brothers and sisters ... "

So we can know that Severus manipulated Mundungus to betray the Order of the Phoenix, and the source of this news is the news that Mundungus got at the meeting of the Order of the Phoenix, so the source of the news must be the Order of the Phoenix.