مُوَطَّأُ مَالِكٍ
Muwatta Malik 399
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ بَلَغَهُ أَنَّ عَبْدَ اللَّهِ بْنَ مَسْعُودٍ، كَانَ يَدِبُّ رَاكِعًا .
Yahya related to me from Malik that he had heard that Abdullah ibn Masud used to move forward while in ruku.
Arabic/English book reference: Book 9, Hadith 68