Thank you, Ceoil!   

 Certainly if an author, like Llewelyn, writing about Eluard, cannot find the source of the quote among Eluard's works, then the needle of probability moves towards Yeats.  But, of course, it would be very comforting, and a service to the innumerable people who cite this phrase, to see if it can actually be sourced anywhere in the Yeats bibliography! 

 Jayintheusa