Allah Almighty said to His Prophet (peace and blessings be upon him): "Take from their wealth a charity by which you purify them and cleanse them, and pray for them; indeed, your prayer is a source of tranquillity for them." [Surah al-Tawbah: 103]
Imam al-Shafi'i (may Allah have mercy on him) said: Sending salah upon them means supplicating for them when taking the charity from them.
Abdullah bin Abi Awfa (may Allah be pleased with him) said: When people brought their charity to the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) would say: 'Allahumma salli ala ali fulan' (O Allah, send Your mercy upon the family of so-and-so). My father also brought his charity, and he (peace and blessings be upon him) said: "O Allah, send Your mercy upon the family of Abu Awfa."
Abu al-Walid narrates from Abdullah bin Abi Awfa (may Allah be pleased with him) that when people brought their charity to the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) would say: "O Allah, send Your mercy upon the family of so-and-so," and then he mentioned the rest of the hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7657