Jami At Tirmidhi
371
Sahih+3

Imran bin Husain narrated:"I asked Allah's Messenger (S) about the Salat for a man who is sitting. He said: 'Whoever performs Salat while standing then that is more virtuous. And whoever performs Salat while sitting, then he gets half the rewards of the one standing, and whoever performs Salat while lying down, then he gets half the rewards of the one sitting

SahihAhmad Muhammad Shakir
SahihAl-Albani