The quote has been misattributed to Maya Angelou at times, including on U.S. postage.
This quote by Joan Walsh Anglund (1967 in her book, A Cup of Sun) has been widely used by Maya Angelou without attribution to Walsh Walsh Anglund, and wrongly misattributed to Maya Angelou many, many times, including on U.S. postage. However, the quote belongs to Joan Walsh Anglund, and is from her book "A Cup of Sun" published in 1967. However, Maya Angelou changed the pronoun "He" to "It" but quoted everything else of Joan Walsh Anglund. Why Maya Angelou never attributed her most famous quote as being Joan Walsh Anglund's is still a mystery to this day.
Source: A Cup of Sun: A Book of Poems (1967)