Sunan Ibn Majah, Chapter: The Chapters on Tribulations

"A man came to the Messenger of Allah (saw) at the first pillar and said: 'O Messenger of Allah, which Jihad is best?' but he kept quiet. When he saw the second Pillar, he asked again, and he kept quiet. When he stoned 'Aqabah Pillar, he placed his foot in the stirrup, to ride, and said: 'Where is the one who was asking?' (The man) said: 'Here I am, O Messenger of Allah.' He said: 'A word of truth spoken to an unjust ruler.'"