Musnad Ahmad 32:1593

Mu'ammal narrated to us, Hammad—meaning Ibn Salamah—narrated to us, Ishaq ibn Abdullah narrated to us, from Anas ibn Malik, that the Prophet (peace and blessings of Allah be upon him) used to meet a man and say: "O so-and-so, how are you?" He would say: "I am well, I praise Allah." The Prophet (peace and blessings of Allah be upon him) would say to him: "May Allah keep you in a state of well-being." One day, the Prophet (peace and blessings of Allah be upon him) met him and said: "How are you, O so-and-so?" He said: "I am well, if I am grateful." He said: "So he remained silent toward him." He said: "O Prophet of Allah, you used to ask me and say: 'May Allah keep you in a state of well-being,' but today you remained silent toward me." He said to him: "I used to ask you and you would say: 'I am well, I praise Allah,' so I would say: 'May Allah keep you in a state of well-being.' But today you said: 'I am well, if I am grateful,' so I doubted, and I remained silent toward you."