Musnad Ahmad 482:152

Hammad ibn Khalid narrated to us, Mu'awiyah ibn Salih narrated to us, from Buhayr ibn Sa'd, from Khalid ibn Ma'dan, from Kathir ibn Murrah, from Uqbah ibn Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The one who recites the Quran aloud is like the one who gives charity aloud, and the one who recites the Quran secretly is like the one who gives charity secretly."