Hasten to deeds before being overtaken by tribulations that are like patches of the darkest night;

The Prophet (may Allah's peace and blessings be upon him) is urging the believer to hasten to good deeds and perform them frequently before they become difficult and he gets preoccupied with tribulations and suspicious matters that would prevent him and turn him away from offering good deeds. During such tribulations, which are as dark as the night, the truth and falsehood are mixed up, and it becomes hard for people to distinguish between them.

Their severity causes one to wander blindly to the extent that he would be a believer in the morning and become a disbeliever in the evening, or he would be a believer in the evening and become a disbeliever in the morning, abandoning his religion for the sake of a transient worldly gain.