Sahih Muslim 402 c
This hadith has been narrated on the authority of Mansur with the same chain of transmitters and he made a mention of this:
" Then he may choose any supplication which pleases him or which he likes."
حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، حَدَّثَنَا حُسَيْنٌ الْجُعْفِيُّ، عَنْ زَائِدَةَ، عَنْ مَنْصُورٍ، بِهَذَا الإِسْنَادِ مِثْلَ حَدِيثِهِمَا وَذَكَرَ فِي الْحَدِيثِ ‏
"‏ ثُمَّ لْيَتَخَيَّرْ بَعْدُ مِنَ الْمَسْأَلَةِ مَا شَاءَ أَوْ مَا أَحَبَّ ‏"‏ ‏.‏