Bilal ibn Rabah was a slave in Makkah. When he believed, his master Umayyah ibn Khalaf would lay him on the burning sand at midday with a heavy rock on his chest, to make him give up Islam. But one word came from Bilal's lips: 'Ahad, Ahad' — One, One.
Abu Bakr bought him and set him free. When the adhan was prescribed in Madinah, the Messenger of Allah ﷺ asked Bilal to call it; his voice was strong and beautiful. The voice that had said 'Ahad' under torture now called the whole city to prayer five times a day.
❝ HadithOne dawn the Messenger of Allah ﷺ asked Bilal: tell me the deed in Islam you most hope in, for I heard the sound of your sandals ahead of me in Paradise. Bilal said: whenever I make wudu, by night or day, I pray with it whatever Allah enables me to pray.Sahih al-Bukhari 1149 · Sahih
A simple habit — prayer after each wudu — and so great its worth.