Musannaf Ibn Abi Shayba 22:320

Qabisah narrated to us, he said: Ibn 'Uyaynah narrated to us, from Mujahid, from al-Sha'bi, from Sa'sa'ah, he said: The first to combine the two tablets and to inherit the kalalah (one who leaves behind no ascendant or descendant heir) was Abu Bakr.