It is narrated on the authority of Zirr ibn Hubaysh that he said,
I asked Ubayy ibn Ka‘b : “Your brother [in faith] Ibn Mas‘ood says, ‘Anyone who stands [for the supererogatory prayer every night] throughout the year will happen to witness the Night of Al-Qadr.’” He (Ubayy) said, “May Allaah have mercy upon him, he wanted to not make the people rely only on it. No doubt, he knew that it is in Ramadhaan, particularly in the last ten nights, and that it is, namely, the 27th night.” Ubayy then took an unrestricted oath (such as without saying, “Allaah willing” in it), that it was the twenty-seventh night. (The sub-narrator asked) “With what do you support that, O Abu Al-Munthir?” He said: “With the portent or the sign, of which the Messenger of Allaah told us, which is that the sun rises brightly the next day and without rays.” [Muslim]
According to another narration of this incident, he said, “The sun rises in the next morning as if it were a round tray and without rays.” [Ahmad]
According to another narration, Ubayy said, “By Allaah, Ibn Mas‘ood knew well that it (the Night of Al-Qadr) is in Ramadhaan, and that it is the twenty-seventh night. But he disliked to tell you that lest you would rely on it [and slacken to do worship at any other time].” [At-Tirmithi]
It is narrated on the authority of Mu‘aawiyah that the Prophet, , said: “The Night of Al-Qadr is the twenty-seventh night [of Ramadhaan].” [Abu Daawood]
It is further narrated on the authority of Ibn ‘Abbaas that a man came to the Prophet, , and said, “O Prophet of Allaah, I am an old sick man and it is too difficult for me to stand (for supererogatory prayers every night). So, please, specify for me a particular night (to stand in prayer on), perchance Allaah would help me witness the Night of Al-Qadr on it.” On that the Prophet, , said: “I advise you [to stand in prayer on] the [twenty] seventh [night of Ramadhaan].” [Ahmad]
Benefits and rulings
First: The righteous predecessors were keen on doing what is good and knowing the seasons which have superiority over the other times so that they would give their utmost effort in worshipping in them.
Second: It is permissible for religious scholars to hide some of their knowledge if this is likely to be beneficial, such as, for example, if they believe that the people would otherwise rely on a particular thing and fall short of doing other righteous deeds.
Third: It is permissible for a Muslim to take an oath to that of which he is fully or almost certain.
Fourth: Among the signs of the Night of Al-Qadr is the rising of the sun in the next morning brightly but without rays.
Fifth: A Muslim has to devote himself to the seasons which have superiority over other times, like the last ten days of Ramadhaan in pursuit of the Night of Al-Qadr, so as to get abundant reward for little work.
Sixth: Researching scholars maintain that the Night of Al-Qadr is not fixed, and that it is most probably on the twenty-seventh night, as Ubayy took an oath to that.
Seventh: The Prophet, , told the old sick man that it was the twenty-seventh night and this does not contradict the other Hadeeths in which it is mentioned that the Night of Al-Qadr is on other nights, for the information given by the Prophet, , about that night is specific to that year in which the man asked him. It is this point of view that reconciles the different texts in that respect.
adopted from islamweb.net