《旧唐书·本纪·卷四》
夏四月十三日，太子太傅、尚书左仆射、燕国公于志宁为太子太师，仍然是同中书门下三品。
夏四月己未，太子太傅、尚书左仆射、燕国公于志宁为太子太师，仍同中书门下三品。
In the 13th day of April, the grand tutor of Crown Prince, the left imperial servant, and the Zhuguogong of the State of Yan, Yu Zhinin, became the grand tutor of Crown Prince, still as the third rank of the Imperial Secretarial Department.