《礼记·曲礼上》
据《曲礼》记载：待人处事必须恭敬严谨，神态应当端庄肃重，好像在沉思似的，所说的话要合理。
《曲礼》曰： 毋不敬，俨若思，安定辞。 
According to the Book of Rites, one must behave respectfully and cautiously, with a solemn expression, as if in deep contemplation, and the words one speaks must be reasonable.