Sayyiduna Anas (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings be upon him) said: "What is that thing which it is not permissible to withhold?" Someone said: Salt. Someone said: Fire. Then when this (question) rendered them unable (to answer), they said: Allah and His Messenger know best. He (peace and blessings be upon him) said: "It is knowledge, which it is not permissible to withhold."