Sayyiduna Amr bin Abasah (may Allah be pleased with him) says: I came to the Messenger of Allah (peace and blessings be upon him) while he (peace and blessings be upon him) was at Ukaz. I said: O Messenger of Allah! Which supplication is more a means of nearness? Or which is the time in which we may attain nearness to Allah or remember Him? He (peace and blessings be upon him) said: "Allah is nearest to His servant in the last part of the night. If you are able, then be among those who remember Allah at that hour."
(b) Sayyiduna Amr bin Abasah (may Allah be pleased with him) says in another narration: I said: O Messenger of Allah! In which part of the night is supplication most accepted? He (peace and blessings be upon him) said: "In the last part of the night."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4663