كتاب الصلاة
4  
The Book of Prayers
(16)
Chapter: The Tashah-hud In The Prayer
(16)
باب التَّشَهُّدِ فِي الصَّلاَةِ ‏
Sahih Muslim 402 b

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."
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 402b
In-book reference : Book 4, Hadith 60
USC-MSA web (English) reference : Book 4, Hadith 794
  (deprecated numbering scheme)