Sunan Abi Dawud, Book 3, Hadith 768 (USC-MSA) ⓘ
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.
حَدَّثَنَا الْقَعْنَبِيُّ، عَنْ مَالِكٍ، قَالَ لاَ بَأْسَ بِالدُّعَاءِ فِي الصَّلاَةِ فِي أَوَّلِهِ وَأَوْسَطِهِ وَفِي آخِرِهِ فِي الْفَرِيضَةِ وَغَيْرِهَا .
also referenced under: Hadith 769 (DarusSalam) & Hadith 768 (Hasan)