Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.012.

It is desirable that the recipient invoke blessing for the zakah payer at the time of its payment, for Allah says: “Take alms of their property that you may purify and sanctify them and pray for them. Verily, your prayers are a comfort for them” [at-Taubah 103]. It is related from ‘Abdullah ibn Abu Awfa that the Messenger of Allah, upon whom be peace, on receiving sadaqah would say: “O Allah, bless the family of Abu Aufa.” This is related by Ahmad and others.

Wa’il ibn Hajr reported that the Prophet, upon whom be peace, prayed for a man who had offered a fine she-camel in his zakah payment: “May Allah bless him and make his camels beneficial to him.” This is related by an-Nasa’i.

Ash-Shaf’i says: “According to this hadith, the leader may pray for the almsgiver upon receiving his payment by saying: ‘May Allah reward you in turn of what you have offered, and may Allah bless what you still possess.'”