It is easy to find the author - Lao Tzu. But I lost hope trying to find the context, e.g. the book of the quote's origin. Hoping Stack Exchange philosophers can provide the answer.
See e.g. Wordsworth Edition with A.Waley translation,1997, page 66.
From a different translation: Lao Tzu, Te-Tao Ching: A New Translation Based on the Recently Discovered Ma-wang-tui Texts (Robert Henricks editor, 1992, Ballantine Books):
The most difficult things in the world begin as things that are easy;
The largest things in the world arise from the minute.