From the first Conference of John Cassian, In a meeting with Abba Moses, the Abba says
…If perchance we are unable to carry out some strict obligation of ours because we are prevented by some good and necessary business, we should not fall into sadness or anger or indignation, which we would have intended to drive out by doing what we omitted. For what is gained by fasting is less than what is spent on anger, and the fruit that is obtained from reading is not so great as the loss that is incurred by contempt of one’s brother.