Musnad Ahmad 27:1417

Qutaybah ibn Sa'id narrated to us, Bakr ibn Mudar narrated to us, from Ibn 'Ajlan, from Wahb ibn Kaysan—and Wahb had met Ibn 'Umar, [this is] not in the book of Ibn Malik—that Ibn 'Umar saw a shepherd of sheep in an ugly place, and Ibn 'Umar had seen a place better than it. Ibn 'Umar said: "Woe to you, O shepherd, move them, for I heard the Prophet, peace and blessings of Allah be upon him, say: 'Every shepherd is responsible for his flock.'"