Mishkat al-Masabih

‘Amr b. Shu'aib, on his father’s authority, told that his grandfather reported God’s Messenger as saying in the course of his sermon, "Observe fully the alliance made in the pre-Islamic period, for it, i.e. Islam, will only make it stronger; but make no new alliances in Islam.” The source is not given. See Tirmidhi, Siyar, 30