Musnad Abu Ya'la al-Mawsili 2:29

Abu Bakr ibn Abi Shaybah narrated to us, Ibn Idris narrated to us, from ‘Asim ibn Kulayb, from his father, from Ibn ‘Abbas, from ‘Umar, who said: "You have certainly known that the Messenger of Allah (may Allah bless him and grant him peace) said: 'Seek it in the last ten [nights], on the odd [nights].'"