Since then, I have done a little more research and found that besides Matthew Henry, it was a saying of his father Philip Henry. Philip, moreover, read the Letters of Samuel Rutherford, and in fact, Rutherford articulated this very expression in a letter dated March 7, 1637, to David Dickson.
Letter CX (March 7, 1637), Letters of Samuel Rutherford (1891 ed.), p. 226:
Now for my case: I think that the council should be essayed, and the event referred to God; -- duties are ours, and events are God's.