Musnad Ahmad 589:3

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Qabisah ibn Dhu'ayb, that Abu Bakr said: "Has anyone among you heard anything from the Messenger of Allah, may Allah bless him and grant him peace, regarding it?" Al-Mughirah ibn Shu'bah stood up and said: "I witnessed the Messenger of Allah, may Allah bless him and grant him peace, decreeing for her the sixth." He said: "Did anyone hear that with you?" Muhammad ibn Maslamah stood up and said: "I witnessed the Messenger of Allah, may Allah bless him and grant him peace, decreeing for her the sixth." So Abu Bakr gave her the sixth.