The Messenger of Allah ﷺ used to exert himself in worship during the last ten nights of Ramadan more than he did at any other time.
حدثنا قتيبة حدثنا عبد الواحد بن زياد عن الحسن بن عبيد الله عن إبراهيم عن الأسود
كان رسول الله ﷺ يجتهد في العشر الأواخر ما لا يجتهد في غيرها
No references are available.
Muslim also narrated it in "itikaf," chapter "Striving hard in the last ten nights of Ramadan," no. 1175, from Qutayba, through him.