Hadith 505110
Chapter 15: The Qur'an - كتاب القرآن
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، أَنَّهُ قَالَ إِنَّمَا أُنْزِلَتْ هَذِهِ الآيَةُ {وَلاَ تَجْهَرْ بِصَلاَتِكَ وَلاَ تُخَافِتْ بِهَا وَابْتَغِ بَيْنَ ذَلِكَ سَبِيلاً} فِي الدُّعَاءِ . قَالَ يَحْيَى وَسُئِلَ مَالِكٌ عَنِ الدُّعَاءِ فِي الصَّلاَةِ الْمَكْتُوبَةِ فَقَالَ لاَ بَأْسَ بِالدُّعَاءِ فِيهَا .
Yahya related to me from Malik from Hisham ibn Urwa that his father said, "The following ayat was sent down about dua - 'And do not be loud in your prayer and do not be silent in it, but seek a way between these.' " (Sura 17 ayat 110) Yahya said that Malik was asked about making dua in obligatory prayers and he said, "There is no harm in making dua in them."
Reference Muwatta Malik 505110
USC-MSA web (English) reference : Book 15, Hadith 39