Hello, I was wondering if anyone could help me identify a song that I think is Rap? or Hip-hop? or maybe R&B? I remember hearing this song on a radio station I think between 2006-2008? I really, really need help finding the name of the song

I have a sample of the melody here *it's just me humming it, sorry for the horrible singing*
If it helps any, I think the melody is played by a guitar or guitar sounding thing.
http://www.4shared.com/music/xA06BYTw/Memo.html