The Prophet (saw) said: "A Muslim cannot acquire anything more beneficial as his embracing Islam than a brother who he benefits from for the sake of Allah." Tanbih al-khawatir, v. 2, p. 179.
Imam Ali (as) said: "Brothers for the sake of Allah, enjoy an enduring amity, due to the firmness of its foundation." Ghurar al-hikam, hadith no 1795.
Imam Ali (as) said: "He who draws close to you for a purpose will abandon you when it is fulfilled." Ghurar al-hikam, hadith no. 8552.