Some caliphs of Egypt, ordered some natives to go on an expedition upstream the Nile [to its sources]; they arrived at a high mountain from the top of which the water rushed down with great roar, so great that no one could hear [the voice of] his companion standing nearby. Then, one of the party decided to climb to the top and see the headwaters and the environs; no sooner had he reached the top than he began dancing, clapping hands and laughing. Then he went roaming on the mountain slopes never to return. His companions never knew what befell him. Whereupon, another climbed up to see and did the same. A third companion climbed but instructed the others to fasten a rope round his waist. Once I shall reach the place where the other two arrived - he said - pull me down lest I do what they did and lose my way back. They followed his instructions. When he reached the top of the mountain, he behaved like the others; then his companions pulled him back, but - it is said - he had become dumb and could not utter a single word. He died soon afterwards. The expedition returned without obtaining any result other than that.

Taken from: books.rafed.net  أخبار نيل مصر

(When speaking about the Nile) And the scholars are unanimously agreed that there is no river in the world longer than the Nile, a month's march in Islam, two months in Nubia, and four months in barren country up to where it emerges from the Moon Mountain behind the equator. There is no river in this world that pours into the Sea of Rum and of China other than the Nile of Egypt. And there is no river in this world that increases and extends in the most heat season when the rivers and springs of the world are short (of water). The higher the temperature, the greater the increase in it . There is no river in this world on which to grow what is grown on the Nile, and from the outflow of a river from the rivers of this world is not levied what is required from the outlet.

Taken from: الفضائل الباهرة في محاسن مصر والقاهرة by ابن ظهيرة

He also said that the Nile flows from under the Sidra of al-Muntaha (1), and that if its trace was tracked, it would be found in it at the beginning of its flow the leaves of Paradise (he said): That is why he recommended eating tilapia from fish, because it follows the leaves of Paradise and grazes them. (2)

(1) Sidra of al-Muntaha: sidra= tree; muntaha= most remote. The tree of the boundary. It is described as a Lote tree that marks the end of the highest point in the heavens, drawing a boundary where nobody and nothing can pass.

(2) Ibn Abd’essalam al-Menoufi (15^th): The flesh of the Djizum (Nile perch) is important, because this fish feeds on herbs: also mentioned by Ibn Zulaq (996).