Argthtjtr

I remember reading something along those lines, perhaps in the De Bello Gallico. Can anyone provide source material to confirm this?

It's probably nonsense, but it's nonsense with an interesting history.

The actual ancient Germanics were herders and farmers, with cattle, barley, and wheat as their staples. Given the natural woodlands of the areas they inhabited, this generally involved slash-and-burn agriculture, which is pretty much the most environmentally-destructive practice within the capabilities of the ancients. This slowly deforested Europe, eventually turning the entire European Plain from forest into farm and ranchland. There is some primeval forest left in Europe, but not much. So, the claim that they were somehow ancient environmentalists is laughable.

Where did this laughable idea come from? It is true that Caesar portrayed them as hunter-gatherers, but he also said they lived mostly off of meat and dairy (if you've never tried "gathering" milk from a wild animal, I strongly recommend you not to try it). Thus, it's clear Caesar didn't have a 100% accurate idea of their lifestyle.

However, it was Renaissance scholars—particularly nationalist German ones (and sympathetic English writers)—who popularized and expanded on the idea that Germanics were hyper-environmentally-aware hunter-gatherers. They invented the idea of the Noble Savage, and applied it to both Germanics and Native Americans (who were also largely farmers, not hunter-gatherers). Some would argue this process never really stopped, and the trope continues today.

Unless specified further it seems quite contestable that Caesar wrote much to this effect.

If any ancient Roman author comes to mind, with sufficient detail, then it is primarily Publius Cornelius Tacitus, namely with his Germania.

In that account we find some descriptions that come a bit closer to "not touching land":

Baduhennna is solely attested by Tacitus's Annals where Tacitus records that a grove in ancient Frisia was dedicated to her, and that near this grove 900 Roman soldiers were killed in 28 CE. Scholars have analyzed the name of the goddess and linked the figure to the Germanic Matres and Matronae.

Soon afterwards it was ascertained from deserters that nine hundred Romans had been cut to pieces in a wood called Baduhenna, after prolonging the fight to the next day, and that another body of four hundred, which had taken possession of the house of one Cruptorix, once a soldier in our pay, fearing betrayal, had perished by mutual slaughter.
74. The Frisian name thus became famous in Germany, and Tiberius kept our losses a secret, not wishing to entrust any one with the war.
WP: Baduhenna

They were helped by a night of bright starlight, reached the villages of the Marsi, and threw their pickets round the enemy, who even then were stretched on beds or at their tables, without the least fear, or any sentries before their camp, so complete was their careless and disorder; and of war indeed there was no apprehension. Peace it certainly was not—merely the languid and heedless ease of half-intoxicated people.
51. Cæsar, to spread devastation more widely, divided his eager legions into four columns, and ravaged a space of fifty miles with fire and sword. Neither sex nor age moved his compassion. Everything, sacred or profane, the temple too of Tamfana, as they called it, the special resort of all those tribes, was levelled to the ground. There was not a wound among our soldiers, who cut down a half asleep, an unarmed, or a straggling foe. The Bructeri, Tubantes, and Usipetes, were roused by this slaughter, and beset the forest passes through which the army had to return

Since fana is Latin for "temples," it has been suggested that it was a temple to a god Tan, shortened from the German word for a pine-tree, Tanne, or that the first element meant "collective."
WP: Tamfana

These are both allusions to the concept of sacred groves that were still in higher currency at the time in Germania when in Roman and Greek lands this concept has 'lost some ground' or popularity. Equally "the land" might be a something like Nerthus or 'Mother Earth'. These woods being 'holy' were then unlikely to be used for agriculture. And some special trees even had a heightened significance, and were later called Irminsul (probably):

In Tacitus' Germania, the author mentions rumors of what he describes as "Pillars of Hercules" in land inhabited by the Frisii that had yet to be explored. Tacitus adds that these pillars exist either because Hercules actually did go there or because the Romans have agreed to ascribe all marvels anywhere to Hercules' credit. Tacitus states that while Drusus Germanicus was daring in his campaigns against the Germanic tribes, he was unable to reach this region, and that subsequently no one had yet made the attempt. Connections have been proposed between these "Pillars of Hercules" and later accounts of the Irminsuls. Hercules was probably frequently identified with Thor by the Romans due to the practice of interpretatio romana.

Adding to the uncertainty present in the question itself, both authors, Caesar and Tacitus are not overly reliable in their accounts of Germanic tribes and people. Therefore the question "Did ancient Germans take pride in leaving the land untouched?" cannot be answered reliably by relying on Roman authors. What can be stated is what they wrote and how they described it. And that might be quite removed from what went on East of the Rhine.

In any case, even the above would suffice in generous interpretation only to sustain that certain designated areas were not to be touched. A general "leaving land untouched" is just incompatible with human activity. That counts as a double if that activity involves drinking beer, for which you need grain.