Gregory the Great — as excerpted in the Catena Aurea on Matthew 20:1-16

“(ubi sup.) Or; To bear the burden and heat of the day, is to be wearied through a life of long duration with the heats of the flesh. But it may be asked, How can they be said to murmur, when they are called to the kingdom of heaven? For none who murmurs shall receive the kingdom, and none who receives that can murmur.”