pardon me for leaving first; excuse me for leaving before you
A set phrase used when departing a workplace, meeting, or social gathering while others remain. It acknowledges that you are leaving ahead of them and shows consideration.
Excuse me for leaving first. Thank you for your hard work.
The meeting is still going on, but I'll be leaving now.
A polite set phrase combining お先に (before others) and 失礼します (excuse me). The exact historical origin is uncertain, but it is a standard expression in modern Japanese business and social etiquette.