This quote from is Calvin and is from the close of his sermon on Luke 1:49-51 in Songs of the Nativity which contains sermons translated by Robert White and was published by Banner of Truth in 2008.
For the rest, let us all beware of the thoughts of our heart and of whatever far exceeds our powers. Such things are thoroughly deceitful. On the contrary, whenever we are tempted to think inflated thoughts – for the devil loves to dazzle us with our own conceits – we must quickly head them off, and in all modesty to declare: “To serve my God is work enough. Supposing I had a much greater store of holiness, I could never fulfil a hundredth part of my duty. So I will aim no higher than God permits: he knows my capacity and my strength. From now on I'll strive to walk in the fear of my God better than before. My life is short and I am still far from the goal he has set. Though I try hard to reach it, the road is long and difficult.”