Musnad Ahmad 33:981

Abu Qatan narrated to us, Hisham narrated to us, from Abu al-Zubayr, from Jabir, who said: "The sun eclipsed during the time of the Messenger of Allah, may Allah bless him and grant him peace, on a day of intense heat. He prayed with his companions and prolonged the standing until they began to collapse. Then he bowed and prolonged it, then he rose and prolonged it, then he bowed and prolonged it, then he rose and prolonged it. Then he prostrated two prostrations. Then he stood and did something similar to that, so there were four bowings and four prostrations."