Musannaf Ibn Abi Shayba 3:2307

Mu'tamir ibn Sulayman narrated to us, from his father, from a man, from 'Ata', that Ibn 'Abbas, Ibn al-Zubayr, Abu Sa'id, and Ibn 'Umar, when they missed a *witr* (an odd-numbered unit of prayer) from the Imam's prayer, they would perform two prostrations.