Musnad Ahmad 1156:176

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah and Hajjaj narrated to us, he said: Shu‘bah narrated to me, he said: I heard Qatadah narrating from Sa‘id ibn al-Musayyib, from ‘Amir, the brother of Umm Salamah, from Umm Salamah: "That the Messenger of Allah (peace and blessings of Allah be upon him) would wake up in a state of janabah (major ritual impurity) without having had a wet dream, then he would fast his day." He said: "So Abu Hurayrah abandoned his fatwa (legal opinion)."