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.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / Q7657