Zirr said: A man said to Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him): Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says about Laylat al-Qadr: Whoever stands (in night prayer) the whole year will find it. Sayyiduna Ubayy (may Allah be pleased with him) said: "May Allah have mercy on Abu Abd al-Rahman; he knows that it is in Ramadan, on the twenty-seventh night, but he wanted people not to rely (on it)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9552