Sayyiduna Anas (may Allah be pleased with him) narrates: the Prophet (peace and blessings be upon him) met a man, and the Prophet (peace and blessings be upon him) asked: "O so-and-so, how are you?" He replied: "I am well; I praise Allah Almighty." The Prophet (peace and blessings be upon him) said to him: "May Allah Almighty keep you well." Then one day that man met the Prophet (peace and blessings be upon him), and the Prophet (peace and blessings be upon him) asked him: "O so-and-so, how are you?" The man replied: "I am well, if I am grateful." The Prophet (peace and blessings be upon him) said nothing to him. He said: "O Prophet of Allah, before, when you asked about me, you said: 'May Allah Almighty keep you well!' But today you have said nothing." The Prophet (peace and blessings be upon him) said to him: "Before, when I asked you, you said: 'I am well, and I praise Allah Almighty,' so I said: 'May Allah Almighty keep you well!' Today you said: 'I am well, if I am grateful,' so you expressed doubt, and therefore I too remained silent."
Ahmad recorded it, and its narrators are the narrators of the Sahih, except for Mu'ammal bin Isma'il, who is thiqah (trustworthy) but has some weakness.