Musnad Abu Ya'la al-Mawsili 2:24

'Abd al-A'la narrated to us, Hammad narrated to us, Yahya bin Sa'id narrated to us, from 'Ubayd bin Hunayn, from Ibn 'Abbas, from 'Umar bin al-Khattab: That the Prophet (may Allah bless him and grant him peace) swore an oath (of abstention) from his wives for a month. When twenty-nine days had passed, he came down to them.