Hadith 503980
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، عَنْ أَبِي أُمَامَةَ بْنِ سَهْلِ بْنِ حُنَيْفٍ، أَنَّهُ قَالَ دَخَلَ زَيْدُ بْنُ ثَابِتٍ الْمَسْجِدَ فَوَجَدَ النَّاسَ رُكُوعًا فَرَكَعَ ثُمَّ دَبَّ حَتَّى وَصَلَ الصَّفَّ .
Yahya related to me from Malik from Ibn Shihab that Abu Umama ibn Sahl ibn Hunayf said, "Zayd ibn Thabit entered the mosque and found the people in ruku, so he went into ruku and then moved slowly forward until he reached the row."
Reference Muwatta Malik 503980
USC-MSA web (English) reference : Book 9, Hadith 67