Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) said: I entered the mosque while the Messenger of Allah (peace and blessings be upon him) was leaving the mosque. I set off behind him (peace and blessings be upon him), and he (peace and blessings be upon him) was not aware of it. After entering a palm grove, he (peace and blessings be upon him) made a long prostration, so that I thought Allah had taken his (peace and blessings be upon him) soul. I came forward and, lowering my head, tried to look at the face of the Prophet (peace and blessings be upon him). He (peace and blessings be upon him) raised his blessed head and asked: "O Abd al-Rahman, what is the matter?" I said: O Messenger of Allah, because of your (peace and blessings be upon him) long prostration I became afraid that Allah Almighty had taken your (peace and blessings be upon him) soul, so I came to see. He (peace and blessings be upon him) said: "When I entered the grove, I met Jibril, and he said: I give you (peace and blessings be upon him) glad tidings that Allah, the Mighty and Majestic, says: Whoever sends salam upon you (peace and blessings be upon him), I will send peace upon him, and whoever sends blessings upon you (peace and blessings be upon him), I will send down mercy upon him."
Imam al-Shafi'i (may Allah have mercy on him) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever forgot to send blessings (upon me) forgot the path to Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19176