At the time of Musa (as) there was a drought. Allah commanded him to to gather all the people in a field outside and pray to Allah for rain. They begged and cried but yet it did not rain.
Musa (as) says to Allah “We have done as you asked but it still did not rain”. Allah says “There is one person in the crowd who did not ask my forgiveness”.
Musa (as) told this to the people and asked them to beg Allah for forgiveness. That person turned back to Allah, asked His forgiveness and then it started to pour with rain!