Not graded
Riyad as-Salihin 1194
وعنها قالت: كان رسول الله صلى الله عليه وسلم ، يجتهد في رمضان ما لا يجتهد في غيره، وفي العشر الأواخر منه، ما لا يجتهد في غيره" ((رواه مسلم)).
'Aishah (May Allah be pleased with her) reported:
The Messenger of Allah (ﷺ) used to strive more in worship during Ramadan than he strove in any other time of the year; and he would devote himself more (in the worship of Allah) in the last ten nights of Ramadan than he strove in earlier part of the month.
[Muslim].
Cited in Riyad as-Salihin (No. 1194).