Musnad Ahmad 25:265

'Amr ibn Muhammad Abu Sa'id al-'Anqazi narrated to us, Sufyan informed us, from Salamah ibn Kuhayl, from 'Imran, from Ibn 'Abbas, who said: "The Messenger of Allah, may Allah bless him and grant him peace, stayed away from his wives for a month. When twenty-nine days had passed, Gabriel came to him and said: 'Your oath has been fulfilled, and the month is complete.'"