In case anyone is coming here from Google, here is the usual way this legend is repeated in copy-and-paste files:
1178: A Chinese document known as the Sung Document records the voyage
of Muslim sailors to a land known as Mu-Lan-Pi (America). Mention of
this document is contained in the publication, The Khotan Amirs, 1933.
I have no idea where this utterly garbled retelling came from, but the original source of this claim is here: Mu-lan-p'i: A Case for Pre-Columbian Transatlantic Travel by Arab Ships by Hui-lin Li.
The source that article uses translates Mu-lan-p'i as Murabit, that is, Morocco and discusses all kinds of bizarre things that can be found there. Indeed, Mu-lan-p'i is very close to "Mu-ra-bi". Hui-lin Li objects that all of the exceedingly curious things in the source can be explained as American products: the "grains of wheat three inches long" are corn; the "six-foot melon" is a pumpkin. Hui-lin Li does not offer his opinion on the "rice and wheat kept in silos", but assures us that such things as "a 'peach' weighing two catties, a 'citron' weighing over twenty catties [12 kg / 27 lb], and a 'lettuce' weighing as much as over ten catties [6 kg / 13 lb]" are all real fruits because "nearly all of [the] accounts of natural products" in this book "were real and identifiable". He supposes, for example, that the "citron" refers to an 30-pound variety of pineapple. In the final paragraph of the article, he backtracks from his entire argument and says that in fact this is a description of Spain and Morocco mixed with little bits of America, and that the information is "necessarily vague and inaccurate in detail."
I personally find this hypothesis slightly unconvincing. Surely proponents of trans-Atlantic contact can do better than this.
Bonus: Did Ibn Farrukh Discover America in 999 CE?
As Buenaventura Bonnet explained in a 1944 Spanish-language journal article, from which I have borrowed the above facts, “The real author of the forged manuscript is none other than Don Manuel Ossuna Saviñón himself” (my translation). Buenaventura Bonnet debunks the story point for point, noting errors of chronology, errors in French library referencing, and more.