The one who deliberately over-exhausts himself will not be rewarded if his aim is hardship, rather he will be rewarded if hardship accompanies an action that he is required to do. That is because hardship is not to be sought in and of itself.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said in Sharh Nuzum Qawaa’id: Once this is established, that Islam does not seek hardship in and of itself, then we should not seek hardship. If an action may be done without hardship, then seeking hardship is not prescribed. For example: if a person says I will go for Hajj on foot so that I may be exhausted in Hajj and my reward will be greater, it should be said to him: Seeking hardship is not prescribed, because the Lawgiver does not intend hardship, so by your action you are going against the aim of the Lawgiver.
If someone were to say: It says in the hadeeth that the Prophet (peace and blessings of Allaah be upon him) said: “Your reward will be commensurate with your effort”, our response is: The meaning of the word effort in this hadeeth does not mean exhausting oneself intentionally, rather what it means is the exhaustion that results from a certain act of worship which the worshipper does not intend to seek. End quote.
The scholars of the Standing Committee for Issuing Fatwas were asked:
What kind of water is it mustahabb to use when doing ghusl from janaabah – cold water or warm water?
They replied: Praise be to Allaah alone and blessings and peace be upon His Messenger and his family and companions.
The Muslim may use warm or cold water, according to what is in his best interests. The matter is broad in scope and the religion of Allaah is easy, as He says (interpretation of the meaning): “Allaah intends for you ease, and He does not want to make things difficult for you” [al-Baqarah 2:185].
And Allaah is the Source of strength. May Allaah send blessing and peace upon our Prophet Muhammad and his family and companions. End quote.
Shaykh ‘Abd al-Azeez ibn Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood
Fataawa al-Lajnah al-Daa’imah (5/328)
And Allaah knows best.