I looked at some articles about Laozi, Taoism and Tea. What I found was the phrase "the froth of liquid jade" might not be due to Laozi, though there are some English Internet articles saying so. What I found was the following:
1) There is no Chinese literature saying Laozi used the phrase 琼浆玉液 to describe tea. Actually, Laozi did like tea, but it was because of its plain taste and medical feel (not the sensational taste as described by the phrase "the froth of liquid jade"). According to some research article (http://cart.ntua.edu.tw/upload/st/200412/200412A03.pdf), Laozi was against food/drinks with strong flavors (五味 = 酸sour﹑甘sweet﹑苦bitter﹑辛spicy﹑咸salty) : 茶的少滋味，正符合《老子》：「五味令人爽」的飲食觀點，所謂「五味令人爽」指五味亂口，使口爽傷之意，此一飲食觀點也與《莊子‧天地》篇所說：「夫失性有五…四曰五味濁口，使口厲爽；五曰趣舍滑心，使性飛揚」的旨趣相同。可見老莊以「無為」為貴，在飲食生活中已涵化為去滋味，依本性的養生實踐。
2) What Laozi commented about tea was a story like this. Laozi was leaving the Hangu Pass, the local officer Yinxi treated him at home with tea, which was the beginning of tea. Laozi said: Those who drink this tea are our disciples. 道经《天皇至道太清玉册》记载：“老子出函谷关，令尹喜迎之于家首献茗，此茶之始。老子曰：食是茶者，皆汝之道徒也。” However, he did not comment on the flavor of tea.
3) The phrase 琼浆玉液 was first used in the Warring States period (476–221BC): 《楚辞·王逸＜九思·疾世＞》：“从卭遨兮栖迟，吮**玉液**兮止渴。”原注：“玉液，琼蘂之精气。” 战国·楚·屈原《招魂》：“华酌既陈，有**琼浆**些。” But Laozi was generally believed to live in the Spring and Autumn period (771-476BC). So the translation of "liquid jade" 玉液could not be attributed to Laozi, according to known Chinese sources. Rather, "froth of liquid jade" seems to describe a kind of bubbling green tea (Matcha). This method was introduced in Song Dynasty (960–1279, see http://en.wikipedia.org/wiki/Matcha).
Therefore, the phrase "froth of liquid jade" might not be from Laozi. And, there is no well-known Chinese record showing the quote. It might actually be a mistranslation, due to the famouseness of Laozi in the Western world.
If you want to cite some Chinese classics about Tea. You can refer to The Classics of Tea (茶经 by 陆羽, http://a2tea.com/luyu/luyu12.php). It's actually the very book that made tea famous in China itself.
After the tea acquired a symbolic and cultural status due to the Classics of Tea, many poets wrote famous quotes about tea. Here are some of them. Hope they are helpful. :)
《答族侄僧中孚赠玉泉仙人掌茶并序》 〖唐〗 李白
《一言到七言诗》 〖唐〗 元稹