Musnad Ahmad 27:1786

Hajjaj narrated to us, from Ibn Jurayj, and Abd al-Razzaq, [who said] Ibn Jurayj informed us, [who said] Abu al-Zubayr informed me, that he heard Ibn Umar saying: The Messenger of Allah (peace and blessings of Allah be upon him) recited: "O Prophet, when you [people] divorce women, divorce them..." [At-Talaq: 1] "...at the beginning of their waiting period."