Musnad Ahmad 31:236

'Abd al-Samad narrated to us, my father narrated to us, and 'Affan narrated to us, 'Abd al-Warith narrated to us, he said: Muhammad ibn Jahadah narrated to us, al-Walid narrated to me, from 'Abdullah al-Bahiy, from Abu Sa'id al-Khudri, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There will be over you rulers to whom hearts will be tranquil and skins will be soft. Then there will be over you rulers from whom hearts will recoil and skins will shudder." A man said: "Shall we fight them, O Messenger of Allah?" He said: "No, as long as they establish the prayer."