Here is the the Zhanguo Ce (戰國策) passage we talked about:
Here is Wu Hong's translation of this passage:
"Once, when the saintly Tseng-tzu lived in the country of Pi, there was another of the same clan who was also called by his name. This one had killed a man, and a neighbour called out to Tseng-tzu's mother, "Tseng Shen has killed a man." His mother did not leave off her weaving but said: 'My son is no murderer.' In a while another cried, 'Tseng Shen killed a man" but she continued to weave as before. The third time someone cried out 'Tseng Shen killed a man' she dropped her shuttle in fear, leaped over the threshold and fled. Despite the virtue of Tseng Shen and his mother's faith in him, when three other had shaken her confidence she too misbelieved her son."
See Wu Hong, The Wu Liang Shrine: The Ideology of Early Chinese Pictorial Art, Stanford University Press, 1989, p. 277.
Wu Hong adds, "This story was widespread during the Han. Sima Qian copied the Zhanguo Ce passage into the Shi Ji, and the saying 'Three men [could make up a fact] that a tiger is in a marketplace; the kind mother dropped her shuttle [and fled]' was frequently mentioned in texts, including the Huainanzi and the Yi Lin."
Here is the text in the Shiji :