Where did "play freebird!" come from?

At Lynyrd Skynyrd concerts in the 70s, the crowd would always exhort the band to "Play Free Bird", the 10-minute guitar anthem that was usually the closing song of the show. This was also immortalized on live albums by the band, and many fans who never actually saw Skynyrd live also became familiar with the phrase. Some smart-aleck fan at some point in the late 70's or early 80's shouted out "Play Freebird" at a concert that WASN'T a Skynyrd show; and like many funny things do, it became a "thing" to do at concerts. Even now, some drunk fool will shout out "Play Free Bird!!" near the end of an Incubus concert.

Hurricain is correct but it was a morning DJ in Chicago, if I remember correctly, who, on the morning of the concerts would encourage people to shout it at any show and it grew from there.

UPDATE:

The other often-given explanation comes from an unlikely place considering the band's Southern roots -- Chicago, Ill. Kevin Matthews, a Chicago radio personality, claims to have originated the whole "Freebird" phenomenon when he called upon his fans, known as KevHeads, to yell the song title out at a Florence Henderson (she played mother Carol on "The Brady Bunch") concert in the late 1980s. KevHeads did their master's bidding, and a tradition was born. Matthews insists that he never intended for it to be yelled at every concert, however. "It was never meant to be yelled at a cool concert -- it was meant to be yelled at someone really lame," he says in an interview with The Wall Street Journal. "If you're going to yell 'Freebird,' yell 'Freebird' at a Jim Nabors concert."

I've never heard that really, but freebird is a song by Lynard skynard that has an insane guitar solo in it. Maybe that has to do with it?