Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) reports this as a marfu' hadith going back to the Prophet (peace and blessings be upon him); he (peace and blessings be upon him) said: "Upon every joint of a human being, giving charity is due every day, and for all of that, the two rak'ahs of Duha suffice."
Al-Tabarani recorded it in al-Mu'jam al-Saghir and al-Mu'jam al-Awsat; its chain contains a narrator whose biography I have not found.