Wisdom from the Early Cynics, Diogenes 17

To Xeniades, who purchased him, Diogenes said, "Come, see that you obey orders." 

When Xeniades quoted the line, "Backward the streams flow to their founts", Diogenes asked, "If you had been ill and had purchased a doctor, would you then, instead of obeying him, have said 'Backward the streams flow to their founts'"? 

Someone wanted to study philosophy under him. Diogenes gave him a fish to carry and told him to follow him. 

And when for shame the man threw it away and departed, some time after on meeting him he laughed and said, "The friendship between you and me was broken by a fish." 

The version given by Diocles, however, is as follows. Someone having said to him, "Lay your commands upon us, Diogenes," he took him away and gave him a cheese to carry, which cost half an obol. 

The other declined; whereupon he remarked, "The friendship between you and me is broken by a little cheese worth half an obol." 

—Diogenes Laërtius, 6.36