Musnad Ahmad 962:49

Sulayman ibn Harb narrated to us, Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Abu Talib al-Dab'i, from Abu Umamah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "That I remember Allah the Exalted from the rising of the sun, and declare His greatness (saying Allahu Akbar), and declare His oneness (saying La ilaha illa Allah), and declare His perfection (saying Subhan Allah), is more beloved to me than that I free four of the children of Ishmael. And that I remember Allah from the afternoon prayer (Asr) until the sun sets is more beloved to me than that I free such and such number of the children of Ishmael."