Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) said: Whoever becomes the guardian of an orphan's wealth and keeps count of it over the years, then when he returns the wealth to him he should inform him that this much zakah is due; if he wishes he may pay the zakah, and if he does not wish he need not pay it.
Imam al-Shafi'i (may Allah have mercy on him) said — in a debate that took place between him and an opponent, his reply concerning this report was: You hold that this is not established from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) on two counts: first, that it is munqati' (broken), and second, that the one who narrated it is not a precise memoriser. The Shaykh said: The reason for its break is that Mujahid did not meet Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and the narrator who is not a precise memoriser is Layth bin Abi Sulaym, whom the scholars of hadith have declared weak. It has also been narrated from Ibn Abbas, except that Ibn Lahi'ah is alone in narrating its chain, and Ibn Lahi'ah is not taken as proof. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7347