Al-Qaasim narrated that the Mother of Believers said:
قُلْتُ يَا رَسُولَ اللَّهِ يَصْدُرُ النَّاسُ بِنُسُكَيْنِ وَأَصْدُرُ بِنُسُكٍ وَاحِدٍ
I said, “O Messenger of Allah! The people return (from Mecca) having done two acts of worship (both Hajj and‘Umrah), but I am coming back with only one.
قَالَ “ انْتَظِرِي فَإِذَا طَهَرْتِ فَاخْرُجِي إِلَى التَّنْعِيمِ فَأَهِلِّي مِنْهُ ثُمَّ الْقَيْنَا عِنْدَ كَذَا وَكَذَا – قَالَ أَظُنُّهُ قَالَ غَدًا – وَلَكِنَّهَا عَلَى قَدْرِ نَصَبِكِ – أَوْ قَالَ – نَفَقَتِكِ ”
He said, “You should wait and when the period of menses is over, you should go to Tan’im and put on lhram and then meet us at such and such time (and I think he said tomorrow). And it [the reward of this ‘Umrah] is according to your hardship”, or he said, “your spending.”
Imam al-Nawawi, in his explanation of Saheeh Muslim, wrote about this hadeeth:
قوله صلى الله عليه وسلم : ( ولكنها على قدر نصبك أو قال : نفقتك ) هذا ظاهر في أن الثواب والفضل في العبادة يكثر بكثرة النصب والنفقة ، والمراد النصب الذي لا يذمه الشرع ، وكذا النفقة .
His (ﷺ) statement “‘And it is according to your hardship’, or he said, ‘your spending’” – It is apparent that the reward and excellence of the (act of) worship increases with an increase in hardship and spending, and the intended meaning of hardship is that which is not dispraised by the sharee’ah, and similarly with spending.
[Al-Minhaaj bi Sharh Saheeh Muslim]