Sayyiduna Ibn Mas'ud (may Allah be pleased with him) said: "Seek Laylat al-Qadr (the Night of Decree) when seventeen nights of Ramadan have passed, the morning of Badr, (the day about which He said:) 'The day of the criterion, the day when the two armies met' [Surah al-Anfal: 41]; and seek it also on the twenty-first and the twenty-third, for it only falls on an odd night."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9042