Musnad Ahmad 429:28

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, from al-Harith ibn Yazid, from al-Harith ibn Abi Dhubab—if Allah wills—from Muhammad ibn Jubayr ibn Mut'im, from his father, that the Messenger of Allah (peace and blessings of Allah be upon him) raised his head toward the sky and said: "The people of Yemen have come to you like pieces of clouds; they are the best of the people of the earth." A man who was with him said to him: "And us, O Messenger of Allah?" He said a hidden word: "Except you."