Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) narrates: I entered the mosque as the Messenger of Allah (peace and blessings be upon him) was going out, so I began walking behind him (peace and blessings be upon him), and he (peace and blessings be upon him) was not aware of my presence, until he (peace and blessings be upon him) entered a date-palm garden, faced the qiblah and prostrated. He (peace and blessings be upon him) prolonged the prostration so much that I feared Allah might have taken his soul. I went closer and bent my head to look at his (peace and blessings be upon him) blessed face. He (peace and blessings be upon him) raised his head and asked: "O Abd al-Rahman, what is the matter with you?" I said: "O Messenger of Allah, you prolonged the prostration so much that I feared you had passed away." He (peace and blessings be upon him) said: "When I entered this garden, I met Jibril (peace be upon him), and he said: 'I give you the glad tidings that Allah Almighty says: Whoever sends salam upon you, I will send peace upon him; and whoever sends salah (blessings) upon you, I will send mercy upon him.'" This hadith is sahih (authentic) on the condition of the two Shaykhs, but they did not record it. In my view there is no hadith more authentic than this regarding the prostration of gratitude.