From Sahih Muslim (Hadith 1344):
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 (ﷺ) uttered La ilaha il-Allah at the end of every prayer
Reference: Muslim 1344