502 – ʿAbd Allāh ibn Abī Awfā - may Allah be pleased with him - reported: “Whenever people brought their alms to the Messenger of Allah ﷺ, he would say, allāhumma ṣalli ʿalayhim (O Allah, send blessings upon them).” Agreed upon. (1)

Vocabulary of the ḥadīth
– allāhumma: meaning “O Allah.” The final -mma substitutes for the vocative particle yā, and therefore the two are never joined; the substitute and the substituted cannot coexist.
– ṣalli ʿalayhim: Lexically, ṣalāh denotes supplication. The wording of that supplication varies according to the condition of the one invoked for; no single fixed formula is required, provided the prayer conveys praise befitting the occasion.

Points derived from the ḥadīth
1 – Allah commanded His Prophet Muḥammad ﷺ to take zakāt from the Muslims and to pray for them when receiving it, saying: “Take alms from their wealth to purify and uplift them thereby, and pray for them; your prayer is a comfort to them. Allah is All-Hearing, All-Knowing.” ⦗9:103⦘. In obedience to that command, whenever a group brought their zakāt to him, he would say, “O Allah, send blessings upon them.”
2 – It is recommended for the one who receives zakāt—whether he does so on behalf of the Muslim community, like the official collectors, or for himself, like a poor person—to supplicate for the payer. Among the transmitted supplications are: “May Allah reward you for what you have given, bless for you what you have kept, and make it a purification for you.”
3 – It is commendable to recompense a benefactor for his kindness, even if only by praying for him, in light of the ḥadīth: “Whoever does you a favor, repay him; if you find nothing with which to repay him, then supplicate for him until you believe that you have…"