What is the complementary part of the noble Prophetic saying: "Whoever wishes that Allah Almighty respond to him during hardships and adversities, let him increase in..."?

The hadith you are asking about is authentic, reported by Tirmidhi and Abu Ya'la, and graded as hasan (good) by Al-Albani. Its wording is: "Whoever wishes that Allah Almighty answers him during hardships and distress should frequently supplicate during times of ease."