It’s true that I haven’t been able to find a single source pointing me to the place where the putative author of today’s quote said exactly these words, even in translation (perhaps someone could help me out). But they are widely attributed to St. Augustine, and certainly embody the spirit of his Easter Reflections, an excerpt from which may be found here.
St. Pope John Paul II, who described Augustine as “an incomparable man whose children and disciples we all are,” cited him often and expounded on these words while speaking in Australia during his 1986 Apostolic Journey to the Far East and Oceania:
We do not pretend that life is all beauty. We are aware of darkness and sin, of poverty and pain. But we know Jesus has conquered sin and passed through his own pain to the glory of the Resurrection. And we live in the light of his Paschal Mystery – the mystery of his Death and Resurrection. “We are an Easter People and Alleluia is our song!”
(see here for the story of this video)