Jami` at-Tirmidhi 793

Zirr said:"I said to Ubayy bin Ka'b: 'O Abu Al-Mundhir! How do you know that it is the night of the twenty-seventh?' He said: 'Rather, the Messenger of Allah informed us that it is a night (after which) the sun rises without rays, so we counted and we remembered it. By Allah! Ibn Mas'ud learned that it is in Ramadan and that it is the nigh of the twenty-seventh, but he did not want to inform you lest you would depend on it