Musnad Ahmad 31:257

Ishaq ibn Isa narrated to us, Hammad ibn Salamah narrated to me, from Bishr ibn Harb, that Ibn Umar came to Abu Sa’id al-Khudri and said: "O Abu Sa’id, have I not been informed that you pledged allegiance to two leaders before the people had gathered upon one single leader?" He said: "Yes, I pledged allegiance to Ibn al-Zubayr, then the people of Syria came and drove me to Hubaysh ibn Duljah, and I pledged allegiance to him." Ibn Umar said: "That is what I feared, that is what I feared"—and Hammad raised his voice while saying it. Abu Sa’id said: "O Abu Abd al-Rahman, have you not heard that the Prophet, peace and blessings of Allah be upon him, said: 'Whoever is able not to sleep a sleep, nor wake a morning, nor reach an evening, except that he is under a leader [should do so]'?" He said: "Yes, but I dislike pledging allegiance to two leaders before the people have gathered upon one single leader."