The Messenger of Allah ﷺ sent me to Yemen, so I said, "Messenger of Allah, you are sending me to a people who are older than me to judge between them."He said, "Go, for Allah Almighty will make your tongue firm and guide your heart."
حدثنا يحيى بن آدم حدثنا إسرائيل عن أبي إسحاق عن حارثة بن مضرب
بعثني رسول الله ﷺ إلى اليمن فقلت يا رسول الله إنك تبعثني إلى قوم هم أسن مني لأقضي بينهم قال اذهب فإن الله تعالى سيثبت لسانك ويهدي قلبك
Its chain of transmission is sahih. It has already appeared with another disconnected chain at 636. Abu Dawud narrated it at length at 327/3 via the route of Simak, from Hanash, from Ali, and Tirmidhi narrated part of it at 277/2 and graded it hasan. It will also appear at 690, and will appear with this chain at 1341.