Musnad Ahmad 28:129

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah narrated to us, from Abdullah ibn Hubayrah, from Abd al-Rahman ibn Murayh al-Khawlani, who said: I heard Abu Qays, the freed slave of Amr ibn al-As, saying: I heard Abdullah ibn Amr saying: "Whoever sends blessings upon the Messenger of Allah (may Allah bless him and grant him peace) once, Allah and His angels will send seventy blessings upon him. So let the servant decrease or increase [that amount]."