What is the explanation of Salamah's Hadith: "A man came to the Prophet (peace and blessings be upon him) asking for food, so he fed him half a wasq of barley. The man, his wife, and their guest kept eating from it until he weighed it. Then he came to the Prophet (peace and blessings be upon him) and said, 'Had you not eaten it, you would have continued to eat from it, and it would have lasted for you,'" and what are the lessons learned from it?