Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: the Prophet (peace and blessings be upon him) was asked about Laylat al-Qadr, and he said: "I had been given knowledge of it, then it slipped away from me. So seek it in the seven or the three nights that remain."
Al-Bazzar recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5050