Related quotes
“Friendship often ends in love; but love in friendship—never.”
Vol. II; LXXXIII
Lacon

“Friendship often ends in love, but love in friendship - never.”

“A friendship that like love is warm;
A love like friendship, steady.”
How shall I woo?
Bartlett's Familiar Quotations, 10th ed. (1919)

Lectures on the English Poets http://www.gutenberg.org/files/16209/16209.txt (1818), Lecture VIII, "On the Living Poets"

“Friendship may, and often does, grow into love, but love never subsides into friendship.”
Quoted by Marguerite Gardiner, Countess of Blessington in Conversations of Lord Byron with the Countess of Blessington http://books.google.com/books?id=w648AAAAYAAJ&q="Friendship+may+and+often+does+grow+into+love+but+love+never+subsides+into+friendship"&pg=PA179#v=onepage (1834).

http://books.google.com/books?id=d8kCAAAAQAAJ&q=%22Because+friendship+is+intercommunication+of+love+therefore+where+love+is+not+mutual+there+can+be+no+friendship%22&pg=PA145#v=onepage
Car l'amitié est un amour mutuel, & s'il n'est mutuel, ce n'est pas amitié.
http://books.google.com/books?id=orIOAAAAQAAJ&q=%22car+l'amiti%C3%A9+est+un+amour+mutuel+%26+s'il+n'est+mutuel+ce+n'est+pas+amiti%C3%A9%22&pg=PA242#v=onepage
Pt. 3, ch. 17
Introduction to the Devout Life (1609)

February 16, 1802
This incident was the subject of Wordsworth's "Alice Fell" http://www.bartleby.com/145/ww190.html.
Diaries

“Oh, call it by some better name,
For friendship sounds too cold.”
Ballads and Songs. Oh, Call It by Some Better Name, st. 1.