Source: Shaheed Murtada Mutahhari, The Narrative of Veracious, Vol.1, STORY, NO.48, p.109-110
Perhaps no one thought that this friendship would ever break-off, and these two friends who were always seen together would part with each other. One of them was well—known to the people by the name of his friend than that of his own name. Whenever the people wanted to talk about either of them, they generally neglected his name and called him “the friend of..."
Yes he had been known by the name of his friend “Imam Sadiq (‘a).” One day as usual he was with Imam (‘a), they entered together in the shoe—makers market. No one expected that their friendship would permanently severe by the time they will come out of the market!?”
He was accompanying the Imam (‘a) as always. They entered the market on that day; his black servant was also with him walking behind him. In the middle of the market, he suddenly turned behind and did not find his servant. After going a few steps he again turned behind but did not see him. For the third time, he turned back and had a look but could not trace his servant, who was busy looking around and lagging behind. For the forth time, he turned behind and finally caught sight of the servant. Getting indignant at him he said: “you son of....! where were you?"
Just as these abusive words came out of his mouth, Imam Sadiq (‘a) amazingly raised his hand, slapped firmly on his forehead and said: “Glory be to Allah, why did you insult his mother? Why did you accuse his mother of an immodest act!? I thought you were a man of piety and God—fearing. But it became apparent to me that you had no piety nor virtue.”
He said: “0 descendant of the Holy Prophet (s)!
This servant is originally from ‘Sind’. And so is his mother.
You know better that they are not Muslims, so the servant’s mother was not a Muslim too. If she were a Muslim, I would not have abused her!”
“Let her mother be a non—Muslim; every race and religion has its own law and tradition in marriage. If they act accordingly to their religious laws and regulations, no fornication is committed, and their children are not illegitimate.” After this statement, the Imam (‘a) said: “from now onwards, stay away from me!”
After this incident, no one saw Imam Sadiq (‘a) walk with him till death overcame one of them and separated them from each other.(1)

NOTES
1. Kafi, v. 2, p. 324 and Wasa’il, v. 2, p. 477.