Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.006A.

In his book, Al-Adab ash-Shar’iah, Ibn Muflih writes that Shaikh Taqiyyuddin Ibn Taymiyyah said: “A credible Jew or Christian who has medical expertise may treat a sick Muslim. Such a person may, likewise, be entrusted with funds or other financial transactions, for Allah, the Exalted, says: ‘Among the People of the Book are some who, if entrusted with a hoard of gold, will (readily) pay it back; others, who, if entrusted with a single silver coin, will not repay it unless you constantly stand over them demanding it’.” (Qur’an 3.75)

We find in a sound hadith that the Prophet, peace be upon him, hired a polytheist as a guide at the time of his migration to Madinah, so he entrusted him with his life and money. The people of the tribe of Khuza’ah, who were both Muslims and non-Muslims, acted as scouts for the Messenger of Allah, peace be upon him. It is also reported that the Prophet, peace be upon him, ordered Muslims to seek treatment from Al-Harith ibn Kaldah, who was a disbeliever. But when a Muslim physician with the expertise is available, one should seek his or her treatment and not turn to anyone else. The same applies when one has to entrust a person with funds or deal with him in business. If a Muslim has to confide in or turn to someone from the people of the Book for medical treatment, he may do so. It is not prohibited to befriend Jews and Christians. And when the Muslim has an opportunity to talk to them, he should address them in ways that are polite and sincere. Indeed, Allah, the Exalted, says: “And do not argue with the people of the Book, except in ways that are best.” Qur’an 29.46

In a hadith on the Treaty of Hudaibiyah, Abu Al-Khattab tells us that “the Prophet, peace be upon him, sent a man of the Khuza’ah tribe to gather intelligence, and the Prophet, peace be upon him, accepted his report despite the fact that he was a disbeliever.” This proves, according to Abu Al-Khattab, that it is quite permissible to take the advice of a non-believing physician for diagnosis and treatment, provided he is not suspect and his fidelity is not doubtful.”