From Sahih Bukhari (Hadith 2070):
Narrated `Aisha:When Abu Bakr As-Siddiq was chosen Caliph, he said, "My people know that my profession was not incapable of providing substance to my family. And as I will be busy serving the Muslim nation, my family will eat from the National Treasury of Muslims, and I will practice the profession of serving the Muslims
Reference: Bukhari 2070