Why the night was taken back
He came out specifically to tell them which one it was.
إِنِّي خَرَجْتُ لأُخْبِرَكُمْ بِلَيْلَةِ الْقَدْرِ، وَإِنَّهُ تَلاَحَى فُلاَنٌ وَفُلاَنٌ فَرُفِعَتْ وَعَسَى أَنْ يَكُونَ خَيْرًا لَكُمُ
Innī kharajtu li-ukhbirakum bi-laylati-l-qadr, wa innahu talāḥā fulānun wa fulānun fa-rufiʿat, wa ʿasā an yakūna khayran lakum
I came out to tell you about the Night of Decree, and so and so and so and so quarrelled, and it was taken up. And perhaps that is better for you.
Where this comes from
Two men arguing cost everybody the exact date. Then he said: look for it in the seventh, the ninth and the fifth. And the last clause turns even that into something to hope in.
The full narration
Narrated 'Ubada bin As-Samit:"Allah's Messenger (ﷺ) went out to inform the people about the (date of the) night of decree (Al-Qadr) but there happened a quarrel between two Muslim men. The Prophet (ﷺ) said, "I came out to inform you about (the date of) the night of Al-Qadr, but as so and so and so and so quarrelled, its knowledge was taken away (I forgot it) and maybe it was better for you. Now look for it in the 7th, the 9th and the 5th (of the last 10 nights of the month of Ramadan)
Source
Ṣaḥīḥ al-Bukhārī · 49
- Agreed upon as authentic (al-Bukhārī / Muslim)