Musnad Ahmad 1156:194

Yahya ibn Sa'id narrated to us, from Ibn Jurayj, who said: 'Abd al-Malik ibn Abi Bakr ibn 'Abd al-Rahman ibn al-Harith ibn Hisham narrated to me, from his father, that he heard Abu Hurayrah saying: "Whoever wakes up in a state of janabah (major ritual impurity) without having had a wet dream, let him not fast." So Abu Bakr and his father 'Abd al-Rahman went until they entered upon Umm Salamah and 'Aishah, and both of them said: "The Messenger of Allah (peace and blessings of Allah be upon him) used to wake up in a state of janabah without having had a wet dream, then he would fast." So Abu Bakr and his father 'Abd al-Rahman went and came to Marwan and told him. Then he said: "I adjure you both, when you go to Abu Hurayrah, to tell him." So they went to Abu Hurayrah and informed him. He said: "Did they both say that to you?" They said: "Yes." He said: "They are more knowledgeable; I was only informed of it by al-Fadl ibn 'Abbas."