Musnad Abu Ya'la al-Mawsili 7:36

Zuhayr narrated to us, Yunus ibn Muhammad narrated to us, Layth ibn Sa’d narrated to us, from Yazid ibn al-Hammad, from ‘Amr, from ‘Abd al-Rahman ibn Huwayrith, from Muhammad ibn Jubayr, from ‘Abd al-Rahman ibn ‘Awf, who said: I entered the mosque and saw the Messenger of Allah (peace be upon him) exiting the mosque, so I followed him, walking behind him, and he did not notice me until he entered a palm grove. He faced the Qibla and prostrated, and he prolonged the prostration while I was behind him, until I thought that Allah had taken his soul. I came walking until I reached him, and I lowered my head to look at his face. He raised his head and said, "What is the matter with you, O ‘Abd al-Rahman ibn ‘Awf?" I said, "When you prolonged the prostration, I thought that Allah might have taken your soul, so I came to look." He said, "When I saw that I had entered the palm grove, I met Gabriel, and he said: 'I bring you glad tidings that Allah says: Whoever greets you, I will greet him, and whoever sends blessings upon you, I will send blessings upon him.'"