When did Mahatma Gandhi say this? (Quote follows.)?

"I cannot teach you violence, as I do not believe in it. I can only teach you not to bow your heads before anyone even at the cost of your life." I would like to use this quote in a paper, but I need to cite the original speech, interview, book, etc. where he was quoted as saying this. I have searched all over the Internet, but I can't find any information. Can anyone help me, please?!?