This specific pose is a fairly recent trope. While knights have been depicted as genuflecting since at least the 13th century:
I know of no medieval illustration where the kneeling figure also has a sword drawn. I would be surprised to see an example of that exact pose pre-dating Tennyson's Idylls of the King.
The earliest image of that type that I'm aware of is John Pettie's The Vigil, dating from 1884:
Even this example isn't exactly what you're referring to, since the sword isn't touching the ground.
The earliest example of the specific trope of the reverent knight genuflecting before God, with his sword planted in the ground before him, of which I'm aware would come from Monty Python and the Holy Grail. I'm not sure if this is the definitive source from which all modern examples derive, but it's the earliest example I can find that is famous enough to be a contender.
Edited to add: Here's a GIF of Arthur assuming the posture in Monty Python and the Holy Grail.
And here's Lancelot doing the same thing in the "You got my note!" scene: