Source: The Narrative of Veracious (2) –by: Ayatollah Murtaza Mutahhari - p.63-65
A man with his son entered Imam Ali’s (a.s) house as his guests. The Imam (a.s) made them sit respectfully in the place of honour, and he (a.s) took his seat opposite them. It was time for meals, and the food was served; after they finished eating; Qanbar, the well-known Imam Ali's (a.s) servant, brought a towel, basin and an ewer for washing the guests' hands. The Imam (a.s) took the ewer from Qanbar and went near to wash the guests' hands. The guest drew back his hands and said: "It is not possible that I stretch my hands so that you wash them.
Ali (a.s) said: "Here is your brother in faith, and your resemblance in creation, without any distinction he is eager to serve you and in turn Allah will recompensate. Why do you want to prevent a good action?
Once again the man refused it, but Ali (a.s) put him under an oath to accept and said: "1 would like to attain the honour of serving my faithful brother in Islam; do not oppose me! The guest complied with it shamefully.
Ali (a.s) said: "I request you to let me wash your hands thoroughly as Qanbar wanted to wash your hands. So, do not feel ashamed, and put all formalities aside”.
When Ali (a.s) finished washing his guest's hands, he (a.s) said to his own honourable son, Muhammad ibn Hanafia: "Wash his son's hands. As I am your father, I washed the father's hands, and you wash the son's hands. If his father had not been here, and only his son was our guest, I would have washed his hands myself. But Allah loves to see that when a father and a son are present in one place, father enjoys a privilege and a priority”.
Muhammad, by the order of his father, stood up and washed the hands of the guest's son.
While narrating this event, Imam Askarl (a.s) said: "A real and faithful Shi'i must behave to". (1)

Notes:
1. Bahar uI-Anwar, v. 9, p. 598.