The Book of Prayers
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، وَابْنُ، بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، بِهَذَا الإِسْنَادِ مِثْلَهُ وَلَمْ يَذْكُرْ " ثُمَّ يَتَخَيَّرُ مِنَ الْمَسْأَلَةِ مَا شَاءَ " .Shu'ba has narrated this on the authority of Mansur with the same chain of transmitters, but he made no mention of this:" Then he may choose any supplication which pleases him
From Sahih Muslim, whose reports the scholars accept as authentic.