Musnad Ahmad 10:8

Abu Salamah Mansur ibn Salamah al-Khuza'i narrated to us, Laith narrated to us, from Yazid ibn al-Had, from 'Amr ibn Abi 'Amr, from Abu al-Huwayrith, from Muhammad ibn Jubayr ibn Mut'im, from 'Abd al-Rahman ibn 'Awf, who said: The Messenger of Allah (peace and blessings of Allah be upon him) went out, and I followed him until he entered a palm grove and prostrated. He prolonged the prostration until I feared—or dreaded—that Allah might have taken his soul—or seized it. He said: So I came to look, and he raised his head and said: "What is the matter with you, O 'Abd al-Rahman?" He said: So I mentioned that to him. He said: Then he said: "Indeed, Gabriel (peace be upon him) said to me: 'Shall I not give you glad tidings that Allah, the Almighty and Majestic, says to you: Whoever sends blessings upon you, I will send blessings upon him, and whoever sends peace upon you, I will send peace upon him?'"