Musnad Ahmad 27:423

Yazid narrated to us, Yahya—meaning Ibn Sa'id—informed us, from Muhammad ibn Yahya ibn Habban, that he was informed by a man, who informed him from his father, Yahya, that he was with Abdullah ibn Umar. And Abdullah ibn Umar said to him during the tribulation (fitna): "Do you not consider killing to be a serious matter?" The Messenger of Allah (peace and blessings of Allah be upon him) said to the three: "Let not two whisper privately to the exclusion of their companion."