From: Anecdotes of the Pious Ones, # 60, v.1
Isa (as) told his companions: "I have a request. If you accept I shall tell you."
The companions said: "We shall accept anything that you say."
Prophet Isa (as) started washing each person's feet. The companions felt very uncomfortable with this gesture, but since they had promised to accept, they submited to his will. When he had finished the task, his companions told him: "You are our teacher/our guide. It is becoming that we wash your feet, not vice versa."
Prophet Isa (as) said: "I did this to make you understand of the fact that an 'Alim (scholar) is much more fit to serve people. I did this due to humility, so that you learn humility, and when you become the guide of people after me, your duty would be serving people with humility. Wisdom grows amidst humility, not with arrogance, just as a seed sprouts in the soft land of the desert as opposed to the hardness of the mountains." 
 Wasail v 2, p 457