Musnad Ahmad 28:278

Hassan ibn Musa narrated to us, Ibn Lahi'ah narrated to us, Abdullah ibn Hubayrah narrated to us, from Ibn Murayh, the freed slave of Abdullah ibn Amr, that he heard Abdullah ibn Amr saying: "Whoever sends one blessing upon the Prophet, may Allah bless him and grant him peace, Allah and His angels will send seventy blessings upon him."