مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2051
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas said: The Messenger of Allah (ﷺ) said: “Seek it [Lailatal Qadr] in the last ten days, on the twenty-ninth or the twenty-fifth or the twenty-seventh.”
Musnad Ahmad 2052lts isnad is saheeh, al-Bukhari (2021)]