Sunan Abi Dawud 21273
In-book no 769حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، قَالَ لاَ بَأْسَ بِالدُّعَاءِ فِي الصَّلاَةِ فِي أَوَّلِهِ وَأَوْسَطِهِ وَفِي آخِرِهِ فِي الْفَرِيضَةِ وَغَيْرِهَا .
Malik said: There is no harm in uttering supplication in prayer, in its beginning, in its middle, and in the end, in obligatory prayer or other.