i've heard it before too, been looking for a source but everyone seems to post it with no source given, seems dodgy at first glance.
i found this:
http://www.ummah.com/forum/showthread.php?250768-Fake-story-about-the-prophet-Muhammad-that-has-become-famous-(bahai-faith-exposed)
http://muslimvillage.com/forums/topic/58467-old-woman-throwing-rubbish/
i read in Sahih Muslim that the Prophet pbuh said:
whoever invents a lie about me, let him reserve his seat in hell.
making up nice stories about the Prophet pbuh only works to discredit him.
i myself have been guilty in the past of repeating baseless stories which i read in story books,
astaghfirullah, may Allah make us firm on the truth even if it is only one word.
Seest thou not how Allah sets forth a parable? - A goodly word like a goodly tree, whose root is firmly fixed, and its branches (reach) to the heavens,
It brings forth its fruit at all times, by the leave of its Lord. So Allah sets forth parables for men, in order that they may receive admonition.
And the parable of an evil Word is that of an evil tree: It is torn up by the root from the surface of the earth: it has no stability.
Allah will establish in strength those who believe, with the word that stands firm, in this world and in the Hereafter;
but Allah will leave, to stray, those who do wrong: Allah doeth what He willeth.
Hast thou not turned thy vision to those who have changed the favour of Allah into blasphemy and caused their people to descend to the House of Perdition?-
Into Hell? They will burn therein,- an evil place to stay in!
And they set up (idols) as equal to Allah, to mislead (men) from the Path!
Say: "Enjoy (your brief power)! But verily ye are making straightway for Hell!"
Quran 14:24-30