Imaam Ahmad narrated this Saheeh (Authentic) Hadeeth on the authority of Abu Qataadah .  In this Hadeeth the Prophet explained that the worst kind of thief is the person who steals from his (own) prayers, meaning he does not complete its Rukoo‘ (bowing), Sujood (prostration) and does not maintain a state of submissiveness and devotion which are the core of prayer.

In this Hadeeth the Prophet explained that there are two kinds of theft - one is known and the other is unknown. He also indicated that the latter type is the worst.

In general, this Hadeeth means that a person may perform prayers but with imperfect submissiveness, Rukoo‘ and Sujood, and consequently the prayer has no effect on the person's manners or behavior.  Moreover, the prayer does not prevent a person from wronging other people or even his own family, or from dishonesty in his work. This happens because this person does not feel the real meaning of the prayer, nor experience the spiritual sweetness of being with Allaah The Almighty. 

We ask Allaah The Almighty to enable us to remember and thank Him frequently and to worship Him perfectly.

Allaah Knows best.