Musnad Ahmad 690:1

'Abd al-Razzaq narrated to us, he said: Sufyan narrated to us, from Mansur, from Salim ibn Abi al-Ja'd, from a man, from Ka'b ibn Murrah al-Bahzi, who said: I asked the Messenger of Allah, peace and blessings of Allah be upon him, "Which part of the night is most answered?" And Sufyan said once: "Most heard." He said: "The last middle of the night. And whoever frees a slave, Allah will free for every limb of his, a limb of his own from the Fire."