Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates: the Prophet (peace and blessings be upon him) commanded voluntary prayer at night and encouraged it, until he said: "Hold fast to the (voluntary) night prayer, even if it is only one rak'ah." Once the Prophet (peace and blessings be upon him) came out and a man was offering voluntary prayer after the congregational prayer had begun, and the Prophet (peace and blessings be upon him) also said: "Will you not desist? Are two prayers to be prayed together?"
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Husayn bin Abdullah, who is da'if (weak).