مُوَطَّأُ مَالِكٍ
Muwatta Malik 511
The Qur'an
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."
Arabic/English book reference: Book 15, Hadith 39