What he said about overdoing it
The warning is that the religion wins.
إِنَّ الدِّينَ يُسْرٌ، وَلَنْ يُشَادَّ الدِّينَ أَحَدٌ إِلاَّ غَلَبَهُ، فَسَدِّدُوا وَقَارِبُوا وَأَبْشِرُوا، وَاسْتَعِينُوا بِالْغَدْوَةِ وَالرَّوْحَةِ وَشَىْءٍ مِنَ الدُّلْجَةِ
Inna-d-dīna yusr, wa lan yushādda-d-dīna aḥadun illā ghalabah. Fa-saddidū wa qāribū wa abshirū, wa-staʿīnū bi-l-ghadwati wa-r-rawḥati wa shayʾin mina-d-duljah
This religion is ease. No one makes it hard on himself without it overcoming him. So aim straight, keep close to the mark, take the good news, and get help from the early morning, the late afternoon, and a little of the night.
Where this comes from
The last clause is travel language. A rider crossing a distance moves in the cool parts and rests in between, and arrives. The one who rides flat out does not.
The full narration
Narrated Abu Huraira: The Prophet (ﷺ) said, "Religion is very easy and whoever overburdens himself in his religion will not be able to continue in that way. So you should not be extremists, but try to be near to perfection and receive the good tidings that you will be rewarded; and gain strength by worshipping in the mornings, the afternoons, and during the last hours of the nights." (See Fath-ul-Bari, Page 102, Vol)
Source
Ṣaḥīḥ al-Bukhārī · 39
- Agreed upon as authentic (al-Bukhārī / Muslim)