Abu Hurayrah (radi Allahu anhu) narrated that the Prophet sal Allahu alayhi wa sallam said: “The strong believer is better and more beloved to Allah than the weak believer, but in both there are good.” (Saheeh Muslim)
The ’strong believer’ does not mean a believer who is physically strong, but rather it means a believer who is strong in his emaan. This is because physical strength can be harmful if it is used to disobey Allah ta’ala. In its essence, physical strength is neither praiseworthy nor blameworthy. If a person uses this strength to benefit his dunya and aakhirah then it becomes praiseworthy. But, if he uses this strength to help himself in disobeying Allah, then it becomes blameworthy.
The strong believer, the one who has strong emaan, will be consistent upon what Allah has made obligatory and he will, with the will of Allah, increase in performing the supererogatory deeds. On the other hand, the weak believer who has a weak emaan will be very negligent in completing the obligatory acts and in leaving of the forbidden.
Then the Prophet (sal Allahu alayhi wa sallam) said, “but in both there are good”, and this means that both the strong believer and the weak believer have good in them. The Prophet (sal Allahu alayhi wa sallam) added this so that the people would not think that there is no good in a believer with weak emaan – rather, the weak believer has good in him, and of this good is that he is, without a doubt, better than the disbeliever.
-Shaykh ibn Uthaymeen rahimahullah, Riyaad as-Saaliheen
Taken from here.