Musnad Abu Ya'la al-Mawsili 154:52

Zuhayr narrated to us, Isma'il bin Ibrahim narrated to us, Khalid informed us, from Abu Qilabah, from Ibn 'Umar, who said: A man called out to the Messenger of Allah (ﷺ) and said: "Which part of the night is the most likely to have a supplication answered?" He said: "The last middle part of the night."