Maliks Muwatta Book 15, Hadith Number 39.

Section : Making Dua.

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.”