Musannaf Ibn Abi Shayba 33:223

'Abdullah ibn Idris narrated to us, from Isma'il ibn Abi Khalid, from Sa'id ibn Abi Burdah, who said: 'Umar wrote to Abu Musa: "To proceed: Indeed, the happiest of shepherds is he whose flock is happy because of him, and the most miserable of shepherds before Allah is he whose flock is miserable because of him. Beware of grazing [freely/greedily], for then your workers will graze, and your likeness before Allah will be like the beast that looked at the greenery of the land and grazed upon it, seeking thereby fatness, while its destruction was in its fatness. And peace be upon you."