Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said to us: "Seek it on the seventeenth of Ramadan, on the twenty-first and on the twenty-third." Then he (peace and blessings be upon him) fell silent.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8543