"It is always darkest just before the day dawneth."
Thomas Fuller, Pisgah Sight (1650), Book II, ch. 2

English Clergyman Thomas Fuller is considered one of the most witty and prolific authors of the 17th century. He was born in 1608, studied at Cambridge and was appointed to the Chapel Royal, Savoy in 1641 (a private chapel of the sovereign associated with the House of Lancaster, which remains a well-known London landmark to this day). He remained there for two years until Oliver Cromwell came to power, and Fuller (a monarchist) left for Oxford.

He returned to London in 1646 and wrote Andronicus, or the Unfortunate Politician, a satire against Cromwell. With the restoration of the monarchy in 1660 he was appointed chaplain-extraordinary to Charles II. During his life he published a number of works noted for their wittiness, anecdotes, epigrams, and puns, such as The Holy State, The Profane State and History of the Worthies of Britain - which was published by his son after his death in 1661.

Confusingly, there are two notable, quotable Thomas Fullers:
The Thomas Fuller quoted above (1608-1661) - an English preacher, historian, and scholar; and Thomas Fuller (1654-1734) - a physician, writer and collector of adages.

This is one of those improving proverbs that are the stock in trade of the contemporary glut of self-help manuals and talking therapies. The darkest hour has long been used figuratively to mean 'the lowest ebb' and there are many such examples of it in print dating from the late 1700s.

The English theologian and historian Thomas Fuller appears to be the first person to commit the notion that 'the darkest hour is just before the dawn' to print. His religious travelogue A Pisgah-Sight Of Palestine And The Confines Thereof, 1650, contains this view:

It is always darkest just before the Day dawneth.

The source of the proverb isn't known. It may be Fuller himself, or he may have been recording a piece of folk wisdom. In 1858, much later than Fuller of course, Samuel Lover attributed the notion to the Irish, in Songs and Ballads:

There is a beautiful saying amongst the Irish peasantry to inspire hope under adverse circumstances:- "Remember," they say, "that the darkest hour of all. is the hour before day."