From Sahih Muslim (Hadith 886):
Abu Huraira reported:A person entered the mosque and said prayer while the Messenger of Allah (ﷺ) was sitting in a nook (of the mosque), and the rest of the hadith is the same as mentioned above, but with this addition:" When you get up to pray, perform the ablution completely, and then turn towards the Qibla and recite takbir (Allah o Akbar =Allah is the Most Great)
Reference: Muslim 886