Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 796
Abdullah b. Mas'ud reported: We were sitting with the Apostle (may peace be upon him) in prayer, and the rest of the hadith is the same as narrated by Mansur He (also said): After (reciting tashahud) he may choose any prayer.