It’s known that concentration, tranquility, and consciousness are pillars of the salah. If a person doesn’t complete the pillars of his salah, he will be stealing from them. This reflects the gravity of such wrongdoing.
The eminent Muslim scholar, Sheikh Yusuf Al-Qaradawi, states: “There is actually a hadith to that effect. It was narrated by many companions who heard it from the Messenger of Allah (peace and blessings be upon him).
Abu-Qatada reported that the Messenger of Allah (peace and blessings be upon him) said, “The worst thief is the one who steals from his salah!” People asked, “O Messenger of Allah, how do they steal from the prayer?” He replied, “He fails to complete its bowings and prostrations.” (Reported by Ahmad, at-Tabarani, Ibn Khuzaymah, and Al-Hakim who proved its authenticity in accordance with the conditions set by Al-Bukhari and Muslim.) This means that hadith is authentic as regards its chain of transmission.
As for its meaning, it is quite clear to anyone who has an insight into religion. Such person condemned by the hadith is deemed the worst of thieves because a thief usually robs other people, while he robs himself! And he steals what should never be stolen: the spirit of the salah, namely submissiveness, serenity and fulfillment of bowing and prostration. There can be no significance in salah without these characteristics.
Allah, the Almighty, says, “Successful indeed are the believers. Those who offer their prayers with all solemnity and full submissiveness.” (Al-Mu’minun: 1-2)”