Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1236
Abu Zubair reported: Abdullah b Zubair used to say La ilaha il-Allah at the end of every prayer like the hadith narrated by Ibn Numair and he reported it in the end, and then reported Ibn Zubair saying: The Messenger of Allah (may peace be upon him) uttered La ilaha il-Allah at the end of every prayer.