John Piper (2016). “A Camaraderie of Confidence: The Fruit of Unfailing Faith in the Lives of Charles Spurgeon, George Müller and Hudson Taylor”, p.21, SPCK
John Wesley (1871). “Wesley his own biographer, being illustrations of his character, labours, and achievements, from his own journals and letters [ed.] with an intr. by G.S. Rowe”, p.158