(a) Al-Zuhri (may Allah have mercy on him) says: I said to Urwah (may Allah have mercy on him): Your brother Abdullah bin al-Zubayr (may Allah be pleased with him) prayed only two rak'ahs in Madinah, like the Fajr prayer. He said: He forgot the Sunnah, or he made a mistake.
(b) Ibn Abbas (may Allah be pleased with them both) says that the Prophet (peace and blessings be upon him) performed four bowings and four prostrations in two rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخسوف / 6306