This story about "Istighfar" is from the life of Imam Ahmed Bin Hanbal, who is considered as a renowned scholar of Islam and a famous theologian. Imam Ahmed is also considered to be the founder of the Hanbali school of fiqh (Islamic jurisprudence) and is one of the most celebrated Sunni theologians, often referred to as the "Sheikh ul-Islam" or the "Imam of Ahl al-Sunnah."
During his old age, while Imam Ahmed was traveling he stopped by a town. After the prayers, he wanted to stay for the night in the masjid yard because he didn't know anyone in the town. Owing to his humility, he hadn't introduced himself to anyone thinking that if he did, he would be welcomed by many people.
Failing to recognize Ahmed bin Hanbal, the caretaker of the mosque refused to let him stay in the mosque. As Imam Ahmed was quite old, the caretaker had to drag him out of the mosque. On seeing this, a baker from a nearby place felt pity for this man (Imam Ahmed) and offered to be the host to him for the night. During his stay with the baker, Imam Ahmed observed that the baker would constantly recite Istighfar (seek forgiveness from Allah). Imam Ahmed asked the baker if the constant practice of saying Istighfar had any effect on him. The baker responded by telling Imam Ahmed that Allah had accepted all of his duas (supplications), except one. When he asked him what dua was it that hadn't been accepted, the baker replied that he had been asking Allah to provide him the privilege to meet the famous scholar Imam Ahmed bin Hanbal.
On this, Imam Ahmed bin Hanbal said that Allah had not only listened to his dua but had dragged him onto his (the baker's) doorsteps. [Summarized from Al Jumuah magazine, vol 19, issue 7]
This story is a reminder of the power of saying Istighfar (seeking forgiveness) frequently.