《旧唐书·本纪·卷三》
夏四月六日，在幽州城南誓师，因而用酒肉款待六军而后派遣他们出发。
夏四月癸卯，誓师于幽州城南，因大飨六军以遣之。
On April 6th, the emperor gave a motivational address to his soldiers to the south of the city of Youzhou, entertained the six armies with wine and food, and then sent them off.