‘Uqbah b. ‘Âmir Al-Juhanî – Allâh be pleased with him – once passed by a man who looked like a Muslim, so he greeted him with salâm, and he replied, “And upon you [be peace] and the mercy of Allâh and His blessings.” A boy informed him, “He is a Christian!” So ‘Uqbah got up and followed the man, caught up with him and said, The mercy of Allâh and His blessings are upon the believers; however, may Allâh give you a long life, and make plentiful your wealth and offspring.” [Al-Bukhârî, Al-Adab Al-Mufrad]

Shaykh Al-Albânî grades its chain of narration hasan in Sahîh Al-Adab Al-Mufrad p444, and notes,

“In this narration this illustrious Companion indicates that it is permissible to supplicate for someone to have a long life, even if he is an unbeliever, and so [this can be done for] a Muslim a fortiori…However, the supplicant should make sure that the unbeliever is not an enemy of the Muslims. It is also taken from this [narration] that such an unbeliever can be commiserated with what has been mentioned therein [i.e. the believers are the ones who will receive the mercy of Allâh and His blessings].”

source: http://www.sayingsofthesalaf.net/index.php/supplicating-for-an-unbeliever/