Musnad Ahmad 1080:3

'Affan narrated to us, Abu 'Awanah narrated to us, from Tariq, from Sa'id ibn al-Musayyib, who said: My father was among those who pledged allegiance to the Prophet, may Allah bless him and grant him peace, under the tree, the Pledge of Ridwan. He said: "We set out the following year for Hajj, and its location became obscure to us. So if it has been made clear to you, then you are more knowledgeable."