مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2531
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
'Umar said: Who knows when Lailatal-Qadr is? Ibn 'Abbas said: The Messenger of Allah (ﷺ) said: “It is in the (last) ten, when seven have passed or seven are left."
Musnad Ahmad 2543Its isnad is saheeh according to the conditions of al-Bukhar