Musnad Ahmad 187:1

Rawh ibn ‘Ubadah narrated to us, he said: Abu Nu‘amah al-‘Adawi narrated to us, from Muslim ibn Budayl, from Iyas ibn Zuhayr, from Suwayd ibn Hubayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "The best of a man’s wealth for him is a fertile mare that produces offspring, or a palm grove that is pollinated." And Rawh said in his house: And it was said to him: "You said to us: I heard the Messenger of Allah, peace and blessings of Allah be upon him." He said: "I heard the Prophet, peace and blessings of Allah be upon him."