Musannaf Ibn Abi Shaybah 4636 — A man who misses part of the imam's

Sayyiduna Abu Sa'id, Sayyiduna Ibn Umar and Sayyiduna Ibn al-Zubayr (may Allah be pleased with them) said about a person who joins the imam in prayer when part of the prayer has already passed him: He should do as the imam does. When the imam completes his prayer, he should make up the part of his prayer that he missed and perform the two prostrations of forgetfulness.