聞之曰 in most cases means a quote is following. But for ancient works like Zhuangzi, it maybe not. Sometimes ancient authors talk with the beginning of 'I heard from others that...' just a style, and the following 'quote' is not from some resource, but just the author's personal opinion, or maybe it is really quote, but the originals has already lost during the long history. As you should know, Zhuangzi is as old as about 2500 years ago, and the quote should be at least 2500 years also, which is a age without paper, pen, very few educated people, and documentation was very hard, so that the lost of the original of a quote is reasonable. Also, in ancient Chinese, there is no quotation mark, even Chinese never used any punctuation marks in writing. Ancient educated people are trained to read the articles without punctuations, and the punctuations are add by modern people. So, please, as a reader of ancient Chinese, never pay that much attention to quotation marks. After all, no matter where the sayings are from, they are always reasonable and worthy to learn.