I am told

Melville habitually borrowed from the writings of others.  In the middle section of White-Jacket (1850), with the frigate Neversink at anchor in the harbor of Rio de Janeiro, Melville increasingly depended on his source-material, according to Howard P. Vincent:

"More than ever, then, Melville turns to his sources, weaving paragraphs from suggestions in Mercier, Leech, and Nicol."  (The Tailoring of Melville's White-Jacket, 116)

As I say, this approach for Melville was habitual.  In his first books Typee and Omoo Melville borrowed extensively from published narratives of South Seas travel and adventure by Charles S.Stewart, David Porter, and William Ellis.  Vincent's The Trying Out of Moby-Dick documents Melville's deep debts to whaling narratives by (among others) Beale, Bennett, and Scoresby.  "Benito Cereno" rewrites the eyewitness narrative of Amasa Delano.   Israel Potter, serialized in Putnam's Monthly Magazine before publication in book form, rewrites Trumbull's pamphlet biography, Life and Remarkable Adventures of Israel R. Potter for a start, then appropriates other sources including biographies of  Benjamin Franklin and John Paul Jones.

Examining Melville's borrowings from Leech in White-Jacket, Vincent noticed that the phrase

" 'they told me' is one of Melville's favorite coverings for literary theft."
(The Tailoring of Melville's White-Jacket, 116)

Variations on this device are "he told me" or "_______ told me" and "I am told" or "I was told."

I find this same device used in "Scenes Beyond the Western Border," to introduce otherwise uncredited borrowings from the journals of Henry S. Turner and topographical engineer William B. Franklin.

Turner, for example, reported a sighting of bees, or bee-like insects:

"July 17  A march of 8 miles this morning brought us across 2 more branches of Horse Creek... A strange insect resembling the honey-bee was seen in swarms:  a perpendicular clay bank on the side of the stream had been perforated in innumerable places by them & when discovered were flying about, & running in & out of the holes they had bored in the surface."  (H. S. Turner, 1845 "Journal of an Expedition"; Letters Received by the Adjutant General, 1822-1860 at Fold3)

Turner's observation is introduced in "Scenes Beyond the Western Border" with a variation on Melville's favored device. 

We passed two bold branches of Horse Creek;  a gentleman told me he saw bees hiving their honey in holes in a clay bank; they are rarely seen so far away from plantations, or from trees.  (May 1853)

 Note too the poetic style employed in the rewrite; instead of "running in and out of the holes" the bees are "hiving their honey in holes."

In similar fashion, two borrowings from William B. Franklin's journal are introduced with the expression,

"I am told."  

Franklin on the Oregon emigrants:

".... those in the rear found the women all crying and the men looking very sad.  Poor people!  they felt that in passing us they broke the last link that bound them to the United States."  (March to South Pass, 22)

The same observation turns up in the September 1852 of "Scenes Beyond the Western Border," introduced with the trusty expression "I am told":

"I am told, that by the time our rear passes their companies, toward what they will ever consider their homes, the women generally are seen to weep.  Heaven help them!"  (554)

Here the writer or ghostwriter appears to forget that two pages earlier, the narrator claimed to have witnessed the sight for himself:

I saw a poor woman weeping. The sight of our return! the home! the friends behind! the wilderness before! (552)

Earlier in the series, in the June 1852 installment,  the narrator's "Imaginary Friend" borrows Franklin's comparison of Scott's Bluff to Stirling Castle, which he introduces with "I am told":

"One view of it, I am told, resembles strongly some picture of Sterling Castle."

(379; corrected to Stirling in Scenes and Adventures in the Army)

In his journal of the 1845 March to South Pass, Franklin saw a resemblance between Scott's Bluff and the Scottish fortress, as follows:

"At the point where we left the river the rock bore a great resemblance to the representations of Stirling Castle."  (March to South Pass, ed. Frank N. Schubert, p11)