Salam and thank you for your question.

We are really unsure of this fact since there is NO authentic narration of the Prophet’s tradition that mentions that explicitly.

However, we have the following tradition (hadith) that implies that Prophet Muhammad (peace be upon him) was the best human who has ever lived. Narrated Abdullah Ibn Mas’ud that the Prophet (peace be upon him) said:

“Allah looked at the hearts of people and found that the heart of Muhammad (peace be upon him) is the best so Allah chose him for His message. Then He looked at hearts of the rest of humans and found that the Prophet’s companions were the best so He chose them to be the delegates of the Prophet and best people to carry the message of Islam. So whoever pious Muslims see as good shall be good with Allah and whoever they see as evil shall be evil with Allah.” (Ahmad)

In another hadith, the Prophet (peace be upon him) said:

“The best generation is my generation, then the generation that follows me, then the generation that comes after that. After those generations, lying will be widespread.” (Al-Shawkani)

We may conclude that the best human who has ever lived is surely Prophet Muhammad (peace be upon him) then his companions are the best people after him, then the two generations that followed them.

The influence of the best of creation affected the moral conduct of those who believed around him so Allah made them the best of people. The next two generations who followed the footsteps of those great people remained steadfast on Allah’s religion and Allah favored them to be second and third best.

May Allah guide us to follow His commandments and the tradition of His beloved Prophet (peace be upon him) so we may become among the best in the sight of Allah inshaAllah.

I hope this helps answer your question.