Majma al-Zawaid — Hadith 5040

Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) narrates that he asked the Prophet (peace and blessings be upon him) about Laylat al-Qadr, and the Prophet (peace and blessings be upon him) said: "It is in Ramadan, so seek it in the last ten nights, for it is an odd night: the twenty-first, or the twenty-third, or the twenty-fifth, or the twenty-seventh, or the twenty-ninth, or the last night. Whoever stands in voluntary prayer in it, seeking it out of faith and hoping for reward, and is then granted it, his past and future sins will be forgiven."
Ahmad and al-Tabarani in al-Mu'jam al-Kabir recorded it. Its chain contains Abdullah bin Muhammad bin Aqil; there is criticism of him, though he has been declared trustworthy.