The last ten nights of Ramadan

Ummul Momineen Sayyidina Aaisha 
(May Allah be pleased with her) reported: 
The Messenger of Allah (Sallallahu Alaihi Wasallam)
used to strive more in worship during Ramadan 
than he (Sallallahu Alaihi Wasallam) strove in any 
other time of the year; and he (Sallallahu Alaihi Wasallam) 
would devote himself more 
(in the worship of ALLAH, Subhanahu wa ta'ala,) in the last 
ten nights of Ramadan than he (Sallallahu Alaihi Wasallam) 
strove in earlier part of the month.
[Muslim].