Hadithcore
Mishkat al-Masabih, 885

Abu Qatada reported God’s Messenger as saying, “The one who commits the worst theft is he who steals from his prayer.” When asked how one could steal from his prayer he replied, “By not performing his bowing and his prostration perfectly.” Ahmad transmitted it.

وَعَنْ أَبِي قَتَادَةَ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَسْوَأُ النَّاسِ سَرِقَةً الَّذِي يَسْرِقُ مِنْ صَلَاتِهِ» . قَالُوا: يَا رَسُولَ اللَّهِ وَكَيْفَ يَسْرِقُ مِنْ صَلَاتِهِ؟ قَالَ: لَا يتم ركوعها وَلَا سجودها ". رَوَاهُ أَحْمد

References1 variant
In-Book Reference
Book 4, Hadith 309