What promise did Babasaheb Tarkhad have to make before his son would agree to travel to Shirdi?

Before his son would agree to accompany his mother to Shirdi, Babasaheb Tarkhad had to make a solemn promise. The son insisted that the family's daily vow must not be broken. According to Chapter 9, he made his father agree that no one in the house would consume their food until the 'Naivedya' had been offered to Sai Baba first. Babasaheb Tarkhad gave his son his word, assuring him, "I will offer Naivedya daily; be certain." Only after receiving this assurance did the son feel at peace to begin his journey.