《新唐书·列传·卷六十四》
假若先去攻打京城，时间必在春季，关东热得早，马容易生病，士兵都想回家，不宜打仗。
若先取京师，期必在春，关东早热，马且病，士皆思归，不可以战。
If we attack the capital first, it will have to be in the spring, and it gets hot early in Guandong, making the horses prone to illness. The soldiers will want to go home and it will not be suitable for battle.