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."