Musnad Ahmad 334:20

'Abd al-Samad narrated to us, he said: 'Umar ibn Rashid al-Yamami narrated to us, he said: Iyas ibn Salamah ibn al-Akwa' al-Aslami narrated to us, from his father, who said: "I never heard the Messenger of Allah, peace and blessings of Allah be upon him, begin a supplication without starting it with: 'Glory be to my Lord, the Most High, the Exalted, the Bestower.'" And Salamah said: I pledged allegiance to the Messenger of Allah, peace and blessings of Allah be upon him, among those who pledged allegiance to him under the tree. Then I passed by him after that while he was with some people, and he said: "Pledge allegiance, O Salamah." I said: I have already done so. He said: "And also," so I pledged allegiance to him a second time.