Musannaf Ibn Abi Shayba 6:769

Hatim ibn Wardan narrated to us, from Yunus, from Ammar, the freed slave of Banu Hashim, who said: I witnessed Umm Kulthum and Zayd ibn Umar; they died in the same hour. They brought them both out, and Sa’id ibn al-Aas prayed over them. He placed Zayd closer to him and placed Umm Kulthum in front of Zayd. Among the people on that day were individuals from the companions of the Prophet, may God bless him and grant him peace, and al-Hasan and al-Husayn were at the funeral.