"Indeed, from the GREATEST of mistakes, which some of those who pray fall into, is abandoning being in the state of ease in the Salaah. Indeed the Prophet (sallallaahu alayhi wa sallam) considered the one who does that, to be from the most EVIL of those who steal.”
The Messenger of Allaah (sallallaahu alayhi wa sallam) stated:
"The most evil of those who steal, is the one who steals from his Salaah."
The companions stated:
"Oh Messenger of Allaah, how does he steal from his salaah?"
The Messenger of Allaah stated:
"He doesn't complete his bowing nor does he complete his prostration."
Or He (sallallaahu alayhi wa sallam) said:
"He doesn't straighten his back in the bowing and prostrating."
(Authenticated by Imaam Al-Albaani in Saheeh Al-Jaami' #987)
The Prophet (sallallaahu alayhi wa sallam) considered stealing from the Salaah more EVIL and SEVERE than the stealing of wealth."
Source: Al-Fawaa-id al-Manthoorah pg. 64