Sahih Bukhari Volume 1, Book 10, Hadith 784

حَدَّثَنَا مُوسَى بْنُ إِسْمَاعِيلَ، قَالَ حَدَّثَنَا هَمَّامٌ، عَنِ الأَعْلَمِ ـ وَهْوَ زِيَادٌ ـ عَنِ الْحَسَنِ، عَنْ أَبِي بَكْرَةَ، أَنَّهُ انْتَهَى إِلَى النَّبِيِّ صلى الله عليه وسلم وَهْوَ رَاكِعٌ، فَرَكَعَ قَبْلَ أَنْ يَصِلَ إِلَى الصَّفِّ، فَذَكَرَ ذَلِكَ لِلنَّبِيِّ صلى الله عليه وسلم فَقَالَ ‏"‏ زَادَكَ اللَّهُ حِرْصًا وَلاَ تَعُدْ ‏"‏‏

Narrated By Abu Bakra : I reached the Prophet in the mosque while he was bowing in prayer and I too bowed before joining the row mentioned it to the Prophet and he said to me, "May Allah increase your love for the good. But do not repeat it again (bowing in that way)."