Musnad Ahmad 27:1229

Abu al-Nadr narrated to us, 'Uqbah—meaning Ibn Abi al-Sahba'—narrated to us, Salim ibn 'Abdullah narrated to us, that 'Abdullah ibn 'Umar narrated to him that he was one day with the Messenger of Allah, peace and blessings of Allah be upon him, along with a group of his companions. The Messenger of Allah, peace and blessings of Allah be upon him, turned to them and said: "O you people, do you not know that I am the Messenger of Allah to you?" They said: "Yes, we bear witness that you are the Messenger of Allah." He said: "Do you not know that Allah has revealed in His Book that whoever obeys me has obeyed Allah?" They said: "Yes, we bear witness that whoever obeys you has obeyed Allah, and that from the obedience of Allah is your obedience." He said: "Indeed, from the obedience of Allah is that you obey me, and from my obedience is that you obey your leaders. Obey your leaders; if they pray sitting, then pray sitting."