Authentic Supplication Number 223.

The Messenger od Allah  said: "When one of you reaches a seated gathering place, then let him petition Salam, then if it becomes him to sit, let him sit. Then when he gets up to leave, let him petition Salam, for the first one is not more deserved that the final one."

Reporters.
At-Tirmithi said it was comely, and Al-Albani agrees. Al-Albani included this hadeeth in The Authentic of Good Sayings as #160.