In "Small Boat, Great Mountain" by Ajahn Amaro,
beginning on p.9 of the text, p.31 of the pdf one finds the following:
There is a story I am reminded of that happened at a Buddhist
conference in Europe. A Tibetan lama was there, and a member
of the audience was an extremely serious German student. The
rinpoche had been teaching visualizations of Tara and the puja
to the 21 Taras. During the course of this teaching, this student,
with great sincerity, put his hands together and asked the question:
“Rinpoche, Rinpoche, I have zis big doubt. You see, all day
we do the puja to the 21 Tara and, you know, I am very committed
to zis practice. I vant to do everything right. But I have zis
doubt: Tara, does she exist or does she not? Really Rinpoche, is
she zhere or not? If she is zhere, I can have a full heart. But if
she’s not zhere, zen I don’t vant to do zhe puja. So please,
Rinpoche, once and for all, tell us, does she exist or does she
not?” The lama closed his eyes for a while, then smiled and
replied, “She knows she is not real.” It is not recorded how the
I suppose you could contact Ajahn Amaro and ask him where he heard this.
By the way, I've always liked this story very much.
In theory, there is no difference between theory and practice. But, in practice, there is.