Moshe Rabbeinu's Descendants Will Return with 10 Tribes

Bamidbar Rabba 16:25:

אף על פי כן לא בטלה גזירתו של הקדוש ברוך הוא שאמר למשה ואעשה אותך העמיד ממנו ששים רבוא שנאמר (ד"ה א כג) ובני רחביה רבו למעלה ולעתיד לבא הקב"ה מכנסן שנאמר (ישעיה מט) הנה אלה מרחוק יבאו והנה אלה מצפון ומים ואלה מארץ סינים והגליות באים עמהם והשבטים שהם נתונים לפנים מן סמבטיון ושלפנים מן הרי חשך הם מתכנסין ובאין לירושלים אמר ישעיה (שם) לאמר לאסורים צאו אלו שנתונים לפנים מן סמבטיון ולאשר בחשך הגלו אלו שנתונים לפנים מן ענן של חשך (שם) ועל דרכים ירעו ובכל שפיים מרעיתם אלו שנתונים בדפנו של אנטוכיא אותה שעה הם נגאלים ובאו לציון בשמחה שנא' (שם נא) ופדויי ה' ישובון וגו

Even so [that Hashem said סלחתי כדברך after the sin of the spies], HKB"H's decree where He told Moshe "And I will make you [into a great nation]" was not nullified. He made stand [descendants] from him [Moshe] 600,000 [actually, more than 600,000 - see the עץ יוסף here and also, see the gemara Berachot 7a at the bottom] as it says "but the sons of Rehabiah were very many". And in the future to come, HKB"H will gather them in as it says, "Behold, these shall come from far; and, lo, these from the north and from the west, and these from the land of Sinim." [The מהרז"ו explains that it seems that the Midrash is missing the fact that the sons of Moshe were exiled to one place, the 10 tribes to another place, and the tribes of Yehuda and Binyamin to another place. He presumably says this due to the 3 words "אלה" "these" in this verse.] And the exiles will come back with them [the descendants of Moshe], and the tribes who were placed toward the inside of the Sambatyon [River] and toward the inside of the Mountains of Darkness will be gathered and come to Yerushalayim. Yeshaya said: "Saying to the prisoners: 'Go forth'" - this refers to those who were placed toward the inside of the Sambatyon [River]. "To them that are in darkness: 'Show yourselves'" - this refers to those who were placed toward the inside of the Cloud of Darkness [see עץ יוסף and מהרז"ו how they treat the change to "Cloud" from "Mountains"]. "They shall feed in the ways, and in all high hills shall be their pasture" - this refers to those who were placed in Antioch. [מהרז"ו is not sure which location the descendants of Moshe Rabbeinu were sent.] At that moment, they will be redeemed and will come to Zion with happiness, as it says, "And the ransomed of the LORD shall return..."