Jamieson, Fausset and Brown on Luke 17:7-10

7-10. say unto him by and by--The "by and by" (or rather "directly") should be joined not to the saying but the going: "Go directly." The connection here is: "But when your faith has been so increased as both to avoid and forgive offenses, and do things impossible to all but faith, be not puffed up as though you had laid the Lord under any obligations to you."