No one thought that this friendship would ever end.  They were always seen together.  One of them was not even personally known to the people.  They called him 'the friend of ..."

        Yes, he was a friend of Imam Ja'far Sadiq (a.s.).  One day, they entered a show market together.  They entered as friends, but by the time they were out of the market, their relations had been permanently severed.

        It so happened that Imam's friend was accompanied by his servant also.  While Imam and he were advancing, the servant leisurely walked behind, looking at the shops and talking to his acquaintance.  His master glanced behind him to find him, but there was no trace of him in sight.  And finally he appeared.

        The master had been provoked and as the servant appeared, he blurted an abuse.  He abused the servant's mother before inquiring where he had been for so long.

        Just as those words came out of his mouth, Imam Ja'far Sadiq (a.s.) regarded his friend with anger and loudly said:
        "Glory be to Allah! You have abused his mother, attributing to her an ignoble act.  I thought you were a man of piety and God-fearing.  It seems you have no semblance of any taqwa."

        The friend said: "O Son of the Prophet!  This servant is from Sind and so is his mother.  You know very well that she was not even a Muslim and therefore if I attributed something ignoble to her, it would be appropriate."

        Imam (a.s.) said: "I know she was a non-Muslim.  But every religion has its own laws and rules.  If a non-Muslim marries according to the laws of his or her religion, no fornication is committed.  Nor are the offspring's illegitimate."

        And then the Imam said: "This is the end of our friendship."
        No one ever saw them together again.
 

Reference:
Pearls of Wisdom.  A String of Incidents in the History of Islam.  The World Federation of Khoja Shia Ithna-Asheri Muslim Communities. Page 46.