According Christies:

We are grateful to Drs. Luuk Pijl for noting that the landscape in this composition derives from that of circa 1509 by Patinir in a private collection, Germany (recorded by M.J. Friedländer, Early Netherlandish Painting, IXb, Leiden and Brussels, 1973, no. 249, pl. 239; published more recently with a clearer illustration, R. Koch, 'Joachim Patinir's 'Meeting of Sts Anthony and Paul in the Wilderness'', The Burlington Magazine, CXXXVIII, March 1996, pp. 181-2, fig. 34).