Yahya related to me from Malik from Hisham ibn Urwa that hisfather said, "The following ayat was sent down about dua - 'And do notbe loud in your prayer and do not be silent in it, but seek a waybetween these.' " (Sura 17 ayat 110)Yahya said that Malikwas asked about making dua in obligatory prayers and he said, "Thereis no harm in making dua in them."
USC-MSA web (English) reference: Book 15, Hadith 39
Arabic reference: Book 15, Hadith 511